Add list-of-available-images to image loading.

When fetching an image that has already been loaded as an image in the
current document, and is still in the memory cache, re-use it directly
without validating its cacheability.

See http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#list-of-available-images

This is consistant with other browsers behaviour.

R=japhet@chromium.org
BUG=354801

Review URL: https://codereview.chromium.org/214623004

git-svn-id: svn://svn.chromium.org/blink/trunk@173127 bbb929c8-8fbe-4397-9dbb-9b2b20218538