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This CL makes |media_controls_container_| only show when user is in
main system menu. This prevents menu from expanding when in sub-menu.

Bug: 1135717
Change-Id: I11d93b61ddc67a3183d18e70b5e64a9b731f61e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453923
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814844}