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}

webrtc/webrtc_examples.gyp

@@ -78,7 +78,7 @@
             'examples/peerconnection/client/peer_connection_client.h',
           ],
           'dependencies': [
-            '../talk/libjingle.gyp:libjingle_peerconnection',
+            'api/api.gyp:libjingle_peerconnection',
             '<(webrtc_root)/system_wrappers/system_wrappers.gyp:field_trial_default',
           ],
           'conditions': [
@@ -364,7 +364,7 @@
           'target_name': 'AppRTCDemo',
           'type': 'none',
           'dependencies': [
-            '../talk/libjingle.gyp:libjingle_peerconnection_java',
+            'api/api.gyp:libjingle_peerconnection_java',
           ],
           'variables': {
             'apk_name': 'AppRTCDemo',