[Media] Add a finch trial param for background MediaSource videos

media/blink/webmediaplayer_impl.h

Index: media/blink/webmediaplayer_impl.h
diff --git a/media/blink/webmediaplayer_impl.h b/media/blink/webmediaplayer_impl.h
index 8e1a16b5e3345d840a758147a99f3979c7a0d7d9..5194a7e1ea5c611ce933e6562944b80f3dfd71b5 100644
--- a/media/blink/webmediaplayer_impl.h
+++ b/media/blink/webmediaplayer_impl.h
@@ -681,9 +681,13 @@ class MEDIA_BLINK_EXPORT WebMediaPlayerImpl
   base::WeakPtr<MediaObserver> observer_;
 
   // The maximum video keyframe distance that allows triggering background
-  // playback optimizations.
+  // playback optimizations (non-MSE).
   base::TimeDelta max_keyframe_distance_to_disable_background_video_;
 
+  // The maximum video keyframe distance that allows triggering background
+  // playback optimizations (MSE).
+  base::TimeDelta max_keyframe_distance_to_disable_background_video_mse_;
+
   // When MSE memory pressure based garbage collection is enabled, the
   // |enable_instant_source_buffer_gc| controls whether the GC is done
   // immediately on memory pressure notification or during the next SourceBuffer