Return an absolute URL for HTMLImageElement.currentSrc

Due to a spec change, currentSrc should return an absolute URL.

BUG=375148

Review URL: https://codereview.chromium.org/290803002

git-svn-id: svn://svn.chromium.org/blink/trunk@174366 bbb929c8-8fbe-4397-9dbb-9b2b20218538