Reduce repetitive EventTarget subclassing

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.

ensureEventTargetData() now returns a reference instead of a
pointer as it can never be NULL.

This patch is based on WebKit r156413 by <akling@apple.com>.

R=tkent
BUG=

Review URL: https://codereview.chromium.org/26878003

git-svn-id: svn://svn.chromium.org/blink/trunk@159393 bbb929c8-8fbe-4397-9dbb-9b2b20218538