gpu_test: enable Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing

      https://codereview.chromium.org/2454153002 requests r4 new revision, and cloud
is ready.

Check https://console.cloud.google.com/storage/browser/chromium-gpu-archive/reference-images/?pli=1&prefix=Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing
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/2471173002
Cr-Commit-Position: refs/heads/master@{#429666}