gpu: Unit tests for IPC command buffer proxy

The command buffer proxy doesn't just forward calls, but it also does
coalescing of flushes, and handles sync token verification. This needs
to be tested for correctness.

R=piman
BUG=824510

Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6802b954e7bc13587ccaaba070f6ae88b0c732de
Reviewed-on: https://chromium-review.googlesource.com/974758
Commit-Queue: Sunny Sachanandani <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Cr-Commit-Position: refs/heads/master@{#545361}
3 files changed