gpu: Check channel before use in CanWaitUnverifiedSyncToken.

The channel may be lost if the gpu process crashes. Command buffer proxy
should always check channel_ before using it. This was found when
enabling gpu scheduler:

https://chromium-swarm.appspot.com/task?id=3698787cf9363510&refresh=10&show_raw=1

R=piman
BUG=514813

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
Change-Id: I24b9aba49863612c57dc07abba4a03f33e1a147f
Reviewed-on: https://chromium-review.googlesource.com/526308
Reviewed-by: Antoine Labour <[email protected]>
Commit-Queue: Sunny Sachanandani <[email protected]>
Cr-Commit-Position: refs/heads/master@{#477518}
2 files changed