Previously, message center was closed only when
WebNotificationTray::ClickedOutsideBubble() was called.
As a result, when a user opens the launcher by Search key,
message center bubble remained open.

This CL also closes the bubble when another window is activated.
The implementation is based on SystemTray::ActivationObserver.

TEST=WebNotificationTrayTest.CloseOnActivation
BUG=757979

Change-Id: Ifea4c2cc56cfe4ca7b0ecfc065426cc30eccc512
Reviewed-on: https://chromium-review.googlesource.com/716441


Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509685}