[Media Notification] Add play pause button

Add a play pause button that will trigger the delegate
to call MediaController and toggle playback.

This also moves the play pause icons from //chrome into
//ui as they are needed in both //chrome and //ash.

BUG=897836

Change-Id: I691695f6ac1ba11bbd17537637d4d66e37f9725b
Reviewed-on: https://chromium-review.googlesource.com/c/1298250
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606246}
10 files changed