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

Committed: https://crrev.com/ec5adec0a9879a31866e98c65ddc7b506b9f49c3
Cr-Commit-Position: refs/heads/master@{#361971}

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

Cr-Commit-Position: refs/heads/master@{#361978}
55 files changed