Have LocalDOMWindow own a set of PostMessageTimer

Now PostMessageTimer in LocalDOMWindow is not owned by anyone and
destroyed only when the timer is fired. It means that if the timer isn't
fired, the timer is not destroyed forever.

This CL fixes this bug by having LocalDOMWindow own generated
PostMessageTimers and all generated timers will be destoryed at the
latest when the LocalDOMWindow is destoryed.

Now LocalDOMWindow refers the window, which causes a circular reference.
This CL also fixes this.

BUG=395010

Review URL: https://codereview.chromium.org/400963002

git-svn-id: svn://svn.chromium.org/blink/trunk@178646 bbb929c8-8fbe-4397-9dbb-9b2b20218538