PausableObject Cleanup - PostMessageTimer

The PostMessageTimer used to send postMessage between same process
documents and uses the kPostedMessage task queue (which is pausable).
There is no need for a separate timer based object to just do a
PostTask.

BUG=907125

Change-Id: I0720a9c2f5f81547aa85be25fea8328146d4244e
Reviewed-on: https://chromium-review.googlesource.com/c/1387671
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621796}
2 files changed