Keep Pixel_DirectComposition_Video_MP4 disabled on Intel, because it
seems to have some sort of bug.

BUG=704389
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/2827833002
Cr-Commit-Position: refs/heads/master@{#467249}