Eagerly unregister PostMessageTimer as a context observer.

The effective lifetime of a PostMessageTimer is over once its callback
fires; unregister as an observer at that point to avoid unnecessary work
during the following GC.

R=haraken
BUG=587012

Review URL: https://codereview.chromium.org/1703883002

Cr-Commit-Position: refs/heads/master@{#375885}
3 files changed