Make canceling Timers fast.

base::Closure recently got an IsCancelled method. Taking advantage of
that the scheduler can short circuit a bunch of logic for cancelled
tasks and avoid running them and the rest of the task selection
machinery.

On the new TimerPerfTest benchmark this makes running 10000 cancelled
tasks aprox 50x - 60x faster (measured on Android and Linux).

Note this patch reverts many of the changes made in
https://codereview.chromium.org/2258713004 in favor of
WeakPtr based cancellation as favored by the base owners.

BUG=605718, 638542

Review-Url: https://codereview.chromium.org/2319053004
Cr-Commit-Position: refs/heads/master@{#417621}
21 files changed