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

webrtc/video/end_to_end_tests.cc

Index: webrtc/video/end_to_end_tests.cc
diff --git a/webrtc/video/end_to_end_tests.cc b/webrtc/video/end_to_end_tests.cc
index 4880b819f49d288ef13cf4fc009f39b44faf12d2..0293f7c110cf1efd2795032fb2ddf043cd775d66 100644
--- a/webrtc/video/end_to_end_tests.cc
+++ b/webrtc/video/end_to_end_tests.cc
@@ -2791,6 +2791,8 @@
     int num_frames_received_ GUARDED_BY(&crit_);
   } test;
 
+  test::ScopedFieldTrials override_field_trials(
+      "WebRTC-VideoContentTypeExtension/Enabled/");
   metrics::Reset();
 
   Call::Config send_config(test.GetSenderCallConfig());
@@ -2875,6 +2877,8 @@
   const bool kEnabledRtx = false;
   const bool kEnabledRed = false;
   const bool kScreenshare = true;
+  test::ScopedFieldTrials override_field_trials(
+      "WebRTC-VideoContentTypeExtension/Enabled/");
   VerifyHistogramStats(kEnabledRtx, kEnabledRed, kScreenshare);
 }