Media Controls: Move to different layer

will-change will force the media controls to use a different
layer. According to Pinpoint this will reduce CC memory usage
by 18% and GPU memory usage by 6%.

BUG=821961,821414

Change-Id: I13c62f79577dfef253acf5f3b89cecaf0c5c1ad3
Reviewed-on: https://chromium-review.googlesource.com/996309Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548524}

third_party/WebKit/Source/modules/media_controls/resources/modernMediaControls.css

@@ -35,6 +35,7 @@ video::-webkit-media-controls {
   justify-content: flex-end;
   align-items: center;
   font-size: 14px;
+  will-change: transform;
 }
 
 audio::-webkit-media-controls-enclosure,