Move ElementData out of Element.h/cpp

These files are way too big!

Review URL: https://codereview.chromium.org/26398005

git-svn-id: svn://svn.chromium.org/blink/trunk@160278 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove RenderingNodeProxy

We introduced RenderingNodeProxy while investigating parallel layout. We've
learned some things from this object, but now it's time to remove it and spend
more time with a whiteboard.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/28913005

git-svn-id: svn://svn.chromium.org/blink/trunk@160279 bbb929c8-8fbe-4397-9dbb-9b2b20218538