Make OffscreenCanvas WebGL(2) context consider taintedness of image source

BUG=696222
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;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

Review-Url: https://codereview.chromium.org/2806803003
Cr-Commit-Position: refs/heads/master@{#464467}
10 files changed