Remove traces of beforeload in HTMLMediaElement

There now remain no places in HTMLMediaElement where events are
dispatched synchronously, so there's no need to protect(this). For the
same reason, the ASSERT is irrelevant, even if it holds true.

BUG=none

Review URL: https://codereview.chromium.org/213343002

git-svn-id: svn://svn.chromium.org/blink/trunk@170123 bbb929c8-8fbe-4397-9dbb-9b2b20218538