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In Webgl's transferToImageBitmap(), we create an ImageBitmap from the
drawingBuffer's mailbox. When calling transferToImageBitmap() twice, the
mailbox is invalid in the second call. In this case, we should return
an ImageBitmap that is the same size as the drawingBuffer but should be
transparent black.

Corresponding layout test has been updated to reflect this case.

BUG=627374
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2139353003
Cr-Commit-Position: refs/heads/master@{#405132}