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According to UnifiedSystemTray UX spec, when a feature pod button has
the corresponding detailed view, it should show arrow icon on the left
of the label to indicate the existence of the detailed view.

As described in the UX spec, it needs custom layout logic.

UX spec: http://shortn/_UobIhDkQUx
Screenshot: http://screen/EogahaoN3m3

TEST=manual
BUG=none

Change-Id: I73b226e7c36c8138310d27fa455133dec421f744
Reviewed-on: https://chromium-review.googlesource.com/1111488
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570308}