Implement FrameMarking header extension support

webrtc/media/engine/webrtcvideoengine.cc

Index: webrtc/media/engine/webrtcvideoengine.cc
diff --git a/webrtc/media/engine/webrtcvideoengine.cc b/webrtc/media/engine/webrtcvideoengine.cc
index 999dac228a4bfa4b3986058bdd0b18b92abf6cfd..2ee56c47bb633b4dfae2f4ba63eb5498134ca976 100644
--- a/webrtc/media/engine/webrtcvideoengine.cc
+++ b/webrtc/media/engine/webrtcvideoengine.cc
@@ -523,6 +523,9 @@ RtpCapabilities WebRtcVideoEngine::GetCapabilities() const {
         webrtc::RtpExtension(webrtc::RtpExtension::kVideoContentTypeUri,
                              webrtc::RtpExtension::kVideoContentTypeDefaultId));
   }
+  capabilities.header_extensions.push_back(
+      webrtc::RtpExtension(webrtc::RtpExtension::kFrameMarkingUri,
+                           webrtc::RtpExtension::kFrameMarkingDefaultId));
   return capabilities;
 }