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We receive a 1x1 frame when the shared window is minimized and will replace it
with a blackframe to avaoid video freezing at last frame. But due to frame size
rounding added later, the 1x1 frame will be dropped. Let's fix it again by
replacing it with a 2x2 black frame.

Bug: 568835
Change-Id: Ifd9a37b62e4f179318c8b30bee4687494d976ab1
Reviewed-on: https://chromium-review.googlesource.com/596920Reviewed-by: Zijie He <zijiehe@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491533}