Enable src= background track switching by default

Bug: 663999
Change-Id: Ib83d981bbb6cefa3de9f3af0ea50c2a99b8d6876
Reviewed-on: https://chromium-review.googlesource.com/c/1406149Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622157}

media/base/media_switches.cc

@@ -238,7 +238,7 @@ const base::Feature kUseAndroidOverlayAggressively{
 
 // Let video track be unselected when video is playing in the background.
 const base::Feature kBackgroundSrcVideoTrackOptimization{
-    "BackgroundSrcVideoTrackOptimization", base::FEATURE_DISABLED_BY_DEFAULT};
+    "BackgroundSrcVideoTrackOptimization", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Let video without audio be paused when it is playing in the background.
 const base::Feature kBackgroundVideoPauseOptimization{