Show overlay cast button on modern media controls

This CL adds some CSS from the legacy controls into the modern media
controls so that the overlay cast button is able to show up.

Bug: 831363
Change-Id: I234079b2b789aad4bdc8c27fb0c124810cd18c17
Reviewed-on: https://chromium-review.googlesource.com/1006070
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549905}