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This is because we use copyTexImage to do the GPU to GPU copying, but in ES3
it is limited that source/destination color format component sizes have to match.

We can lift this limitation once we implement a different path where shaders are
used to do the copying instead of copyTexImage.

BUG=429053
TEST=gpu_bots
R=kbr@chromium.org,cwallez@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2040773002
Cr-Commit-Position: refs/heads/master@{#398230}