Add shadow to overlay play button

This CL adds a shadow to the overlay play button so that it's visible
against white backgrounds.

Bug: 831032
Change-Id: If9af93e25efe8a1e1b3cf802629a15c3e2d0597e
Reviewed-on: https://chromium-review.googlesource.com/1004112Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549594}

third_party/blink/renderer/modules/media_controls/resources/modernMediaControls.css

@@ -267,6 +267,7 @@ input[pseudo="-webkit-media-controls-overlay-play-button" i]::-internal-media-co
   background-position: center center;
   background-image: -webkit-image-set(url(ic_play_arrow.svg) 1x);
   background-color: rgba(255, 255, 255, .9);
+  box-shadow: 0 0 20px 0 rgba(0, 0, 0, 0.10);
 
   transition: opacity cubic-bezier(0.4, 0.0, 0.6, 1) 600ms;
 }