Add field trial for balanced degradation preference.

webrtc/media/engine/webrtcvideoengine.cc

Index: webrtc/media/engine/webrtcvideoengine.cc
diff --git a/webrtc/media/engine/webrtcvideoengine.cc b/webrtc/media/engine/webrtcvideoengine.cc
index 999dac228a4bfa4b3986058bdd0b18b92abf6cfd..9a2865b9c574c5ec4084bf517aeda011c9eceb21 100644
--- a/webrtc/media/engine/webrtcvideoengine.cc
+++ b/webrtc/media/engine/webrtcvideoengine.cc
@@ -1742,6 +1742,9 @@ WebRtcVideoChannel::WebRtcVideoSendStream::GetDegradationPreference() const {
   } 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;
     }