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

webrtc/video/video_send_stream.cc

Index: webrtc/video/video_send_stream.cc
diff --git a/webrtc/video/video_send_stream.cc b/webrtc/video/video_send_stream.cc
index f8a83ea89b06c8374de15e76cf17c78865413076..7bf32c6dbaa2cef8712346409f1b55b53e13d9bf 100644
--- a/webrtc/video/video_send_stream.cc
+++ b/webrtc/video/video_send_stream.cc
@@ -828,6 +828,10 @@ VideoSendStreamImpl::VideoSendStreamImpl(
     RTC_DCHECK_GE(id, 1);
     RTC_DCHECK_LE(id, 14);
     RTC_DCHECK(RtpExtension::IsSupportedForVideo(extension));
+    if (StringToRtpExtensionType(extension) == kRtpExtensionVideoContentType &&
+        !field_trial::IsEnabled("WebRTC-VideoContentTypeExtension")) {
+      continue;
+    }
     for (RtpRtcp* rtp_rtcp : rtp_rtcp_modules_) {
       RTC_CHECK_EQ(0, rtp_rtcp->RegisterSendRtpHeaderExtension(
                           StringToRtpExtensionType(extension), id));