Remove uses of #pragma once and add PRESUBMIT check.

They violate the C++ coding style guide:
https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md#File-headers

BUG=webrtc:7191
NOTRY=True

Review-Url: https://codereview.webrtc.org/2707843002
Cr-Commit-Position: refs/heads/master@{#16712}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 3405ef9..f7500e8 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -169,6 +169,26 @@
   return []
 
 
+def _CheckNoPragmaOnce(input_api, output_api):
+  """Make sure that banned functions are not used."""
+  files = []
+  pattern = input_api.re.compile(r'^#pragma\s+once',
+                                 input_api.re.MULTILINE)
+  for f in input_api.AffectedSourceFiles(input_api.FilterSourceFile):
+    if not f.LocalPath().endswith('.h'):
+      continue
+    contents = input_api.ReadFile(f)
+    if pattern.search(contents):
+      files.append(f)
+
+  if files:
+    return [output_api.PresubmitError(
+        'Do not use #pragma once in header files.\n'
+        'See http://www.chromium.org/developers/coding-style#TOC-File-headers',
+        files)]
+  return []
+
+
 def _CheckNoFRIEND_TEST(input_api, output_api):
   """Make sure that gtest's FRIEND_TEST() macro is not used, the
   FRIEND_TEST_ALL_PREFIXES() macro from testsupport/gtest_prod_util.h should be
@@ -514,6 +534,7 @@
       input_api, output_api))
   results.extend(_CheckNativeApiHeaderChanges(input_api, output_api))
   results.extend(_CheckNoIOStreamInHeaders(input_api, output_api))
+  results.extend(_CheckNoPragmaOnce(input_api, output_api))
   results.extend(_CheckNoFRIEND_TEST(input_api, output_api))
   results.extend(_CheckGnChanges(input_api, output_api))
   results.extend(_CheckUnwantedDependencies(input_api, output_api))
diff --git a/webrtc/examples/peerconnection/client/conductor.h b/webrtc/examples/peerconnection/client/conductor.h
index 726534b..0041a9b 100644
--- a/webrtc/examples/peerconnection/client/conductor.h
+++ b/webrtc/examples/peerconnection/client/conductor.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_CONDUCTOR_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_CONDUCTOR_H_
-#pragma once
 
 #include <deque>
 #include <map>
diff --git a/webrtc/examples/peerconnection/client/defaults.h b/webrtc/examples/peerconnection/client/defaults.h
index ecfb9ba..f4527c0 100644
--- a/webrtc/examples/peerconnection/client/defaults.h
+++ b/webrtc/examples/peerconnection/client/defaults.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_DEFAULTS_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_DEFAULTS_H_
-#pragma once
 
 #include <stdint.h>
 
diff --git a/webrtc/examples/peerconnection/client/flagdefs.h b/webrtc/examples/peerconnection/client/flagdefs.h
index 92e2773..19a2b02 100644
--- a/webrtc/examples/peerconnection/client/flagdefs.h
+++ b/webrtc/examples/peerconnection/client/flagdefs.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_FLAGDEFS_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_FLAGDEFS_H_
-#pragma once
 
 #include "webrtc/base/flags.h"
 
diff --git a/webrtc/examples/peerconnection/client/main_wnd.h b/webrtc/examples/peerconnection/client/main_wnd.h
index d61f95d..01c8823 100644
--- a/webrtc/examples/peerconnection/client/main_wnd.h
+++ b/webrtc/examples/peerconnection/client/main_wnd.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_MAIN_WND_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_MAIN_WND_H_
-#pragma once
 
 #include <map>
 #include <memory>
diff --git a/webrtc/examples/peerconnection/client/peer_connection_client.h b/webrtc/examples/peerconnection/client/peer_connection_client.h
index dbf2d8f..de460dd 100644
--- a/webrtc/examples/peerconnection/client/peer_connection_client.h
+++ b/webrtc/examples/peerconnection/client/peer_connection_client.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_PEER_CONNECTION_CLIENT_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_CLIENT_PEER_CONNECTION_CLIENT_H_
-#pragma once
 
 #include <map>
 #include <memory>
diff --git a/webrtc/examples/peerconnection/server/data_socket.h b/webrtc/examples/peerconnection/server/data_socket.h
index 0ef61ea..6350dcf 100644
--- a/webrtc/examples/peerconnection/server/data_socket.h
+++ b/webrtc/examples/peerconnection/server/data_socket.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_DATA_SOCKET_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_DATA_SOCKET_H_
-#pragma once
 
 #ifdef WIN32
 #include <winsock2.h>
diff --git a/webrtc/examples/peerconnection/server/peer_channel.h b/webrtc/examples/peerconnection/server/peer_channel.h
index 6fd740d..3d51db2 100644
--- a/webrtc/examples/peerconnection/server/peer_channel.h
+++ b/webrtc/examples/peerconnection/server/peer_channel.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_PEER_CHANNEL_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_PEER_CHANNEL_H_
-#pragma once
 
 #include <time.h>
 
diff --git a/webrtc/examples/peerconnection/server/utils.h b/webrtc/examples/peerconnection/server/utils.h
index e1c8729..fd875f5 100644
--- a/webrtc/examples/peerconnection/server/utils.h
+++ b/webrtc/examples/peerconnection/server/utils.h
@@ -10,7 +10,6 @@
 
 #ifndef WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_UTILS_H_
 #define WEBRTC_EXAMPLES_PEERCONNECTION_SERVER_UTILS_H_
-#pragma once
 
 #include <assert.h>
 #include <string>