Reland of Protect new header extension by field trial experiment to allow hardcoding it in SDP

webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
index 3f4c401a19d5c43ff9f64c6a9e3bd14d60ff9fb6..129cee5ca9b8f8a7be723d40b4543e8a1012e9f1 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
@@ -28,7 +28,7 @@
 #include "webrtc/modules/rtp_rtcp/source/rtp_format_vp9.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_header_extensions.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_packet_to_send.h"
-
+#include "webrtc/system_wrappers/include/field_trial.h"
 namespace webrtc {
 
 namespace {
@@ -328,7 +328,8 @@
       last_rotation_ = current_rotation;
       // Report content type only for key frames.
       if (frame_type == kVideoFrameKey &&
-          video_header->content_type != VideoContentType::UNSPECIFIED) {
+          video_header->content_type != VideoContentType::UNSPECIFIED &&
+          webrtc::field_trial::IsEnabled("WebRTC-VideoContentTypeExtension")) {

[danilchap, 2017/06/07 08:55:11]

May be instead check for the experiment in
webrtc::VideoSendStreamImpl::VideoSendStreamImpl:
 skip call to rtp_rtcp->RegisterSendRtpHeaderExtension if the extension is video
content type and not in the experiment.

[ilnik, 2017/06/07 15:23:14]

Done.

         last_packet->SetExtension<VideoContentTypeExtension>(
             video_header->content_type);
       }