content/gpu: Simplify stub scheduling.

Get rid of the reschedule timeout in GpuScheduler. Also let the channel
know which stub is scheduled/descheduled so that it can map it to the
correct stream.

BUG=514813

Review URL: https://codereview.chromium.org/1348363003

Cr-Commit-Position: refs/heads/master@{#349788}
8 files changed