Replace Timers used in ActiveDOMObject with AsyncMethodRunner

AsyncMethodRunner is a wrapper of Timer with simplified interface
for running methods asynchronously. It's mainly for use in
ActiveDOMObject subclasses.

Where suspend() and resume() are not implemented assuming other code
suspends timers, implement them to ensure they work even when those
external code doesn't suspend timers correctly.

BUG=none

Review URL: https://codereview.chromium.org/27571005

git-svn-id: svn://svn.chromium.org/blink/trunk@160316 bbb929c8-8fbe-4397-9dbb-9b2b20218538