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Instead of using a fixed frame size 320x240, use a size that is
supported by the device.

BUG=chromium:881300
TEST=run capture_unittests on kevin
TEST=run capture_unittests with another fix in chromium:881300
     on amd64-generic VM

Change-Id: I3a9ec8b781675b05f6a98d0221ae9dbb0a9c4694
Reviewed-on: https://chromium-review.googlesource.com/1249469Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595481}