Fixing WebRTC after moving from src/webrtc to src/

In https://webrtc-review.googlesource.com/c/src/+/1560 we moved WebRTC
from src/webrtc to src/ (in order to preserve an healthy git history).
This CL takes care of fixing header guards, #include paths, etc...

NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
TBR=tommi@webrtc.org


Bug: chromium:611808
Change-Id: Iea91618212bee0af16aa3f05071eab8f93706578
Reviewed-on: https://webrtc-review.googlesource.com/1561
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19846}
diff --git a/api/datachannel.h b/api/datachannel.h
index 28112ad..70be913 100644
--- a/api/datachannel.h
+++ b/api/datachannel.h
@@ -8,11 +8,11 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#ifndef WEBRTC_API_DATACHANNEL_H_
-#define WEBRTC_API_DATACHANNEL_H_
+#ifndef API_DATACHANNEL_H_
+#define API_DATACHANNEL_H_
 
 // Including this file is deprecated. It is no longer part of the public API.
 // This only includes the file in its new location for backwards compatibility.
-#include "webrtc/pc/datachannel.h"
+#include "pc/datachannel.h"
 
-#endif  // WEBRTC_API_DATACHANNEL_H_
+#endif  // API_DATACHANNEL_H_