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/pc/mediastreamtrack.h b/pc/mediastreamtrack.h
index 0fb2de2..33ffbe2 100644
--- a/pc/mediastreamtrack.h
+++ b/pc/mediastreamtrack.h
@@ -8,13 +8,13 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#ifndef WEBRTC_PC_MEDIASTREAMTRACK_H_
-#define WEBRTC_PC_MEDIASTREAMTRACK_H_
+#ifndef PC_MEDIASTREAMTRACK_H_
+#define PC_MEDIASTREAMTRACK_H_
 
 #include <string>
 
-#include "webrtc/api/mediastreaminterface.h"
-#include "webrtc/api/notifier.h"
+#include "api/mediastreaminterface.h"
+#include "api/notifier.h"
 
 namespace webrtc {
 
@@ -59,4 +59,4 @@
 
 }  // namespace webrtc
 
-#endif  // WEBRTC_PC_MEDIASTREAMTRACK_H_
+#endif  // PC_MEDIASTREAMTRACK_H_