Block load event dispatching on old document when an HTMLMediaElement is moved between documents.

      BUG=272786
TEST=LayoutTests/http/tests/media/video-in-iframe-crash.html

Review URL: https://codereview.chromium.org/25362002

git-svn-id: svn://svn.chromium.org/blink/trunk@159031 bbb929c8-8fbe-4397-9dbb-9b2b20218538