Drop all events fired at documents or windows that have script disabled

In the effort to remove ChildNodeLazySnapshot I realized we were creating
events and then dispatching them at svg images inside ::insertedInto.

Instead we should just ignore all events that are on documents that can't
run script anyway. This allows eventually adding a NoEventDispatchAssertion
to the ChildNodeInsertionNotifier.

Review URL: https://codereview.chromium.org/226743006

git-svn-id: svn://svn.chromium.org/blink/trunk@171099 bbb929c8-8fbe-4397-9dbb-9b2b20218538