Disable background video track behind a feature flag

BUG=663999
TEST=unit test + manual testing with the flag on - see chrome://media-internals logging track selection

Review-Url: https://codereview.chromium.org/2487373003
Cr-Commit-Position: refs/heads/master@{#434255}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 8a9cd15..36e0cb1 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2005,6 +2005,11 @@
      IDS_NEW_AUDIO_RENDERING_MIXING_STRATEGY_DESCRIPTION,
      kOsWin | kOsMac | kOsLinux | kOsAndroid,
      FEATURE_VALUE_TYPE(media::kNewAudioRenderingMixingStrategy)},
+    {"disable-background-video-track",
+     IDS_BACKGROUND_VIDEO_TRACK_OPTIMIZATION_NAME,
+     IDS_BACKGROUND_VIDEO_TRACK_OPTIMIZATION_DESCRIPTION,
+     kOsAll,
+     FEATURE_VALUE_TYPE(media::kBackgroundVideoTrackOptimization)},
 #if defined(OS_CHROMEOS)
     {"files-quick-view", IDS_FLAGS_FILES_QUICK_VIEW_NAME,
      IDS_FLAGS_FILES_QUICK_VIEW_DESCRIPTION, kOsCrOS,