Move talk/app/webrtc to webrtc/api

The previously disabled warnings that were inherited from
talk/build/common.gypi are now replaced by target-specific disabling
of only the failing warnings. Additional disabling was needed since the stricter
compilation warnings that applies to code in webrtc/.

License headers will be updated in a follow-up CL.

Other modifications:
* Updated the header guards.
* Sorted the includes using chromium/src/tools/sort-headers.py
  except for these files:
  talk/app/webrtc/peerconnectionendtoend_unittest.cc
  talk/app/webrtc/java/jni/androidmediadecoder_jni.cc
  talk/app/webrtc/java/jni/androidmediaencoder_jni.cc
  webrtc/media/devices/win32devicemanager.cc

The HAVE_SCTP define was added for the peerconnection_unittests target
in api_tests.gyp.

I also checked that none of
SRTP_RELATIVE_PATH
HAVE_SRTP
HAVE_WEBRTC_VIDEO
HAVE_WEBRTC_VOICE
were used by the talk/app/webrtc code.

For Chromium, the following changes will need to be applied to the roll CL that updates the
DEPS for WebRTC and libjingle:
https://codereview.chromium.org/1615433002

BUG=webrtc:5418
NOPRESUBMIT=True
R=deadbeef@webrtc.org, pthatcher@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1610243002 .

Cr-Commit-Position: refs/heads/master@{#11545}
diff --git a/webrtc/api/objc/RTCVideoSource+Private.h b/webrtc/api/objc/RTCVideoSource+Private.h
index 2300848..c363d33 100644
--- a/webrtc/api/objc/RTCVideoSource+Private.h
+++ b/webrtc/api/objc/RTCVideoSource+Private.h
@@ -10,7 +10,7 @@
 
 #import "RTCVideoSource.h"
 
-#include "talk/app/webrtc/videosourceinterface.h"
+#include "webrtc/api/videosourceinterface.h"
 
 NS_ASSUME_NONNULL_BEGIN