Remove beforeload events.

This feature makes the insertedInto tree walk much more complicated and
is the source of security bugs. We've had it disabled behind a flag for a
while now and not seen complaints or bugs from users.

BUG=333318

Review URL: https://codereview.chromium.org/205523003

git-svn-id: svn://svn.chromium.org/blink/trunk@169994 bbb929c8-8fbe-4397-9dbb-9b2b20218538