Don't assume correct image format in CopyVpxImageToVideoFrame()

The data arriving inside the |vpx_image| struct is not guaranteed by
libvpx to be in one of the formats supported by VpxVideoDecoder.

BUG=569574
TEST=Loading http://shion.ru/crash.webm should result in video decoding error

Review URL: https://codereview.chromium.org/1520313002

Cr-Commit-Position: refs/heads/master@{#365213}
2 files changed