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

webrtc/video/video_send_stream_tests.cc

Index: webrtc/video/video_send_stream_tests.cc
diff --git a/webrtc/video/video_send_stream_tests.cc b/webrtc/video/video_send_stream_tests.cc
index de3da25385e5f463108cfde683920b7b22aea77a..340c71c7bfc78896a6f5714897d65229744bb2de 100644
--- a/webrtc/video/video_send_stream_tests.cc
+++ b/webrtc/video/video_send_stream_tests.cc
@@ -332,6 +332,8 @@ TEST_F(VideoSendStreamTest, SupportsVideoContentType) {
     }
   } test;
 
+  test::ScopedFieldTrials override_field_trials(
+      "WebRTC-VideoContentTypeExtension/Enabled/");
   RunBaseTest(&test);
 }