Overlay expected and actual repaint rects for LayoutTests.

Based on dsinclair@'s original patch https://codereview.chromium.org/174073002/

For the new world of text based repaint tests it can be hard
to visualize how the repaint rects appear on the screen. This
is a first pass at visualizing the difference between the old
and new repaint rects when one of the text based repaint tests
fail.

Review URL: https://codereview.chromium.org/303223008

git-svn-id: svn://svn.chromium.org/blink/trunk@175182 bbb929c8-8fbe-4397-9dbb-9b2b20218538