Add field trial for balanced degradation preference.

webrtc/media/engine/webrtcvideoengine2.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 1733 matching lines @@
   // |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 {
       degradation_preference = DegradationPreference::kMaintainFramerate;
+      if (webrtc::field_trial::IsEnabled("WebRTC-Video-BalancedDegradation"))

[mflodman1, 2017/06/13 09:23:19]

WDYT of, to avoid more nested if statements:

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;
}

[åsapersson, 2017/06/13 10:17:15]

Done.

+        degradation_preference = DegradationPreference::kBalanced;
     }
   }
   return degradation_preference;
 }
 
 const std::vector<uint32_t>&
 WebRtcVideoChannel2::WebRtcVideoSendStream::GetSsrcs() const {
   return ssrcs_;
 }
 
@@ skipping 912 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