- Pause monitoring if there is no callback. It would be wasteful to
  schedule tasks to check memory usage if the monitor doesn't have
  to notify. Also remove Start() method as it is no longer needed.
- Add cool down interval for the next monitoring when a near-OOM is
  detected. This prevent us from triggering the intervention
  aggressively. For example, without cool down interval, we may trigger
  the intervention just after a page navigation if we triggered the
  intervention on the previous page.

Bug: 776665
Change-Id: Ia01f40d4a85165f4dbe679abb2ddc2323d49e869
Reviewed-on: https://chromium-review.googlesource.com/753564


Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514363}