[Media Controls] Reland "Remove overlay play button on desktop"

This is a reland of crrev.com/c/1252846, modified to enable test mode
on a flaky layout test.

Description from crrev.com/c/1252846:

This CL updates the media controls on desktop to have the standard
play button in the bottom left corner instead of the overlay play
button in the center.

As part of this change, the play button is given a disabled state
when appropriate, which also applies to the audio player. This fixes
a separate issue where the audio player wasn't giving proper feedback
with a broken source.

Bug: 890078, 839101
Change-Id: I03fa0a56b34c91cddeb9884a7922dc6885c3ac70
Reviewed-on: https://chromium-review.googlesource.com/c/1268663
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597673}
229 files changed