Remove ImageDiff code and update code to run image_diff from chromium instead

This removes ImageDiff and updates scripts to instead rely on the image_diff target
which is defined in the chromium repo.

Depends on https://codereview.chromium.org/18872002/

R=dpranke

Review URL: https://chromiumcodereview.appspot.com/18441006

git-svn-id: svn://svn.chromium.org/blink/trunk@155141 bbb929c8-8fbe-4397-9dbb-9b2b20218538