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This CL closes UnifiedSystemTray when other widgets are activated.
With this CL, we can also remove some CloseBubble() that manually
closes UnifiedSystemTray.

TEST=manual
BUG=864247

Change-Id: I7ef09aec5bb79213ee87407d2fe04179847968f7
Reviewed-on: https://chromium-review.googlesource.com/1140096
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576405}