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- introduce MessageCenterScrollView and use for
MessageCenterView::scroller_. We need this because the |scroller_| is
the one that gets focused when a user invokes Alt+Shift+N.

- remove accessibility bits from MessageCenterView including accessible
data and focusability.

Test: with ChromeVox, press Alt+Shift+N and tab around. Verify that the
scroller gets focused and outputs correctly when first entering message
center and that tabbing never reaches the MessageCenterView, but only
reaches the scroller_. Previously, both were tab stops.

Bug: 
Change-Id: I1c8b06f157bbb4749f61cdd6a4f80c5693d76f48
Reviewed-on: https://chromium-review.googlesource.com/801344
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521130}