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This CL changes button row layout tree so that play/pause
button is always centered regardless the visibility of previous/next
track button.

Bug: 1010645
Change-Id: Iba274ccd820346885768fa7243ff523313df3632
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834899
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702606}