MessagePumpCFRunLoopBase's approach to timers, as recommended by Apple,
leads to two timer reschedulings per timer firing. Timer reschedulings
are quite expensive, and the extra rescheduling can cause Chrome to
consume up to 2x the CPU of Safari on the same page. This change uses
private (but not secret) API to temporarily invalidate a
MessagePumpCFRunLoopBase's CFRunLoopTimer, causing the run loop to bypass
the extra rescheduling.

R=mark@chromium.org, rsesek@chromium.org
BUG=511406

Review-Url: https://codereview.chromium.org/2709813003
Cr-Commit-Position: refs/heads/master@{#452852}