Move throttling of background timers into the renderer scheduler

Not only does this simplify the code, it's more efficent since
previously setting the timer alignment resulted in mass cancellation
and reposting of timers.

BUG=510398, 546953, 560402

Review URL: https://codereview.chromium.org/1441073006

Cr-Commit-Position: refs/heads/master@{#361971}
55 files changed