Permit ActiveDOMObject to update the ExecutionContext it observes.

HTMLMediaElement and HTMLMarqueeElement are the only two instances of an ActiveDOMObject where they can be reparented into a new ExecutionContext. As such, the ActiveDOMObject should be updated to observe the new ExecutionContext and the state synchronized.

BUG=346497
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/178743005

git-svn-id: svn://svn.chromium.org/blink/trunk@167935 bbb929c8-8fbe-4397-9dbb-9b2b20218538