Simplify rate limiter logic in Canvas2DLayerBridge

This CL is a follow-up to https://codereview.chromium.org/2653933003/.
It refactors the rate-limiter code to make it stop using it own task
observer, in favor of piggy-backing on top of the finalize frame signal

BUG=653548,673469

Review-Url: https://codereview.chromium.org/2700833002
Cr-Commit-Position: refs/heads/master@{#452071}
2 files changed