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The layout_gravity value center does the same thing as
center_vertical|center_horizontal. This CL makes use of center.

Change-Id: I625ffe5dc5b147ed39d986d862f38ee432fbfa7a
Reviewed-on: https://chromium-review.googlesource.com/1102909Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567779}