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This CL removes reference to runtime flags from MessageListView,
and moved the option to the constructor.
Direct reference to flag led to unit test failure in
MessageCenterViewTest.SizeAfterUpdateOfRepositionTarget.

TEST=out/Release/ash_unittests '--gtest_filter=*MessageCenter*' \
--enable-features=SystemTrayUnified
BUG=847104

Change-Id: I82d33c827d79bf5fc53d30453e8ac91b18396356
Reviewed-on: https://chromium-review.googlesource.com/1088329Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564948}