scheduler: Throttle timers in out-of-view frames

Throttle timer tasks to 1 Hz in out-of-view or invisible frames. This
reduces the power consumption and performance impact of these tasks,
which are not likely having any visible effect on the page anyway.

BUG=616519

Review-Url: https://codereview.chromium.org/2023033003
Cr-Commit-Position: refs/heads/master@{#415278}
12 files changed