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Unified system tray had been launched long time ago.
|use_unified_theme_| is always true now.

Bug: 1131543
Change-Id: I38ceb731b02e774a8f8ee44577077cdeb5103f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508635
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823701}