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This CL removes WebNotificationTray button when --enable-features=
SystemTrayUnified.
UnifiedSystemTray will replace WebNotificationTray and SystemTray.
When the flag is enabled, UiController, AshPopupAlignmentDelegate, and
MessagePopupCollection will be also owned by UnifiedSystemTray.

TEST=manual
BUG=828752

Change-Id: I03cf6949bb2b165d4bda89327570aac74328e88c
Reviewed-on: https://chromium-review.googlesource.com/987635Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548837}