Update pixel test expectation and add new worker tests

In the previous CL: https://codereview.chromium.org/2328463004/ where
the commit() function is implemented, we add a new pixel test and put its
expectation to fail. Now that the reference image has been generated on
all GPU waterfall bots, it is time to update the expectation.

This CL also adds a new test to make sure that WebGL's commit()
is behaving as expected on a worker thread.

BUG=563852
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/2358833002
Cr-Commit-Position: refs/heads/master@{#420304}
3 files changed