Add field trial for balanced degradation preference.

webrtc/media/engine/webrtcvideoengine.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 1724 matching lines @@
   // Do not adapt resolution for screen content as this will likely
   // result in blurry and unreadable text.
   // |this| acts like a VideoSource to make sure SinkWants are handled on the
   // correct thread.
   DegradationPreference degradation_preference;
   if (!enable_cpu_overuse_detection_) {
     degradation_preference = DegradationPreference::kDegradationDisabled;
   } else {
     if (parameters_.options.is_screencast.value_or(false)) {
       degradation_preference = DegradationPreference::kMaintainResolution;
+    } else if (webrtc::field_trial::IsEnabled(
+                   "WebRTC-Video-BalancedDegradation")) {
+      degradation_preference = DegradationPreference::kBalanced;
     } else {
       degradation_preference = DegradationPreference::kMaintainFramerate;
     }
   }
   return degradation_preference;
 }
 
 const std::vector<uint32_t>&
 WebRtcVideoChannel::WebRtcVideoSendStream::GetSsrcs() const {
   return ssrcs_;
@@ skipping 914 matching lines @@
         rtx_mapping[video_codecs[i].codec.id] !=
             ulpfec_config.red_payload_type) {
       video_codecs[i].rtx_payload_type = rtx_mapping[video_codecs[i].codec.id];
     }
   }
 
   return video_codecs;
 }
 
 }  // namespace cricket