SVGImage should allow event dispatch since there's never any listeners

We can create a special exception when there's a NoEventDispatchAssertion
above the SVGImage loading code since we know SVGImage is not capable
of actually running script.

This is in preparation of locking down ChildNodeInsertionNotifier to
have a NoEventDispatchAssertion and a ScriptForbiddenScope.

Review URL: https://codereview.chromium.org/296933011

git-svn-id: svn://svn.chromium.org/blink/trunk@174699 bbb929c8-8fbe-4397-9dbb-9b2b20218538