MacViews: Enable GPU rasterization (where available)

This should relieve pressure on the browser's UI thread.

Disallow anonymous GLImages when GpuMemoryBuffers are disabled
(several tests would attempt to use this path while it was
disabled, and crash).

Add a NOTREACHED to cc::TestImageFactory, so that such failures
are easier to diagnose in the future.

Bug: 845213
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ibaf017e9ee3c22b3391fe2d2b47b82c8fbff8e93
Reviewed-on: https://chromium-review.googlesource.com/1076531
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562696}
4 files changed