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This CL makes the GMC toolbar button appear when there is an active
Cast session. GMC dialog contents for Cast will be added in a future CL.

The functionality is put behind a kGlobalMediaControlsForCast feature
flag.

Bug: 987479
Change-Id: Ie97f8d1e9ca6cd94fc982d2d60c490aa777499d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873200
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710158}