GMC: Reland "Add interactive UI tests"

This CL relands the interactive UI tests for the global media controls.
The original CL was flaky for the ShowsMetadataAndControlsMedia test
because sometimes it checked for toolbar icon visibility after media
started but before the media starting caused the icon to appear. This
CL fixes that by waiting for the icon to appear.

TESTED=Was locally flaking about once every 20 runs before the fix.
After the fix, I ran it 300 times and it passed every time.

Description from original CL (crrev.com/c/1753690):
This CL adds some basic interactive UI tests for the global media
controls. This ensures we have better coverage than just unit tests.

Bug: 993879, 999297
Change-Id: I7abf36e4f82b64b262d3d0a7eb9d426e8ae1c5f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814671
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702882}