<https://webkit.org/b/121915>

Reviewed by Antti Koivisto.

Add a EventTargetWithInlineData helper class to EventTarget.h.
It's a simple subclass with an EventTargetData member and FINAL
eventTargetData()/ensureEventTargetData() overrides.

Turns out that this fits almost every use case except Node and
SVGElementInstance.

Inlined most of the FINAL EventTarget overrides since they will
be called on tight pointers by generated bindings code and this
removes the need for virtual dispatch.

Spammed OVERRIDE FINAL as appropriate, we can't make the whole
classes FINAL until bug 121747 is fixed, so added FIXMEs.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156413 268f45cc-cd09-0410-ab3c-d52691b4dbfc