Remove some failure entry in gpu pixel test expectations

In a previous CL: https://codereview.chromium.org/2365653005/, we fixed
gpu pixel test for WebGL's commit API on Mac, so we mark those tests as
failure. We have also added two tests to transferToImageBitmap API in the
same CL. Now that the CL has been picked up by all GPU bots, and I have
checked all reference images, it is time to remove the failure entry.

TBR=kbr@chromium.org, zmo@chromium.org, bajones@chromium.org
BUG=649668
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2376533003
Cr-Commit-Position: refs/heads/master@{#421235}