Make prettydiff work again

r159595 and r159600 both delete code that was used by PrettyDiff.
This restores that code.

R=dpranke@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/26662005

git-svn-id: svn://svn.chromium.org/blink/trunk@159619 bbb929c8-8fbe-4397-9dbb-9b2b20218538