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This CL has DummyView handle a back key event and close the Chrome OS
message center if it opens. This fix is for ag/112017890.

Note that DummyView handles key events only when no other ARC window
is focused.

ARC-side: ag/4779503
Chromium-side: crrev.com/1175709

Bug: b/112017890
Test: Ran atest
Change-Id: If0603bd68f2ad4f482521e7cf3e698edb934183f
Reviewed-on: https://chromium-review.googlesource.com/1175709Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590044}