DevTools: added image node and URL to ImageDecode and ImageResize events.

Remember ImageRenderObject being painted for the time of paint and use it to
retrieve generating node id and resource URL when we encounter decode/resize.
This only works for main thread decode/resize so far, deferred ones require more
work.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23523021

git-svn-id: svn://svn.chromium.org/blink/trunk@157221 bbb929c8-8fbe-4397-9dbb-9b2b20218538