Compute minimum repaint and show them in overlay

For repaint tests, compute minimum repaint by comparing pixel dump
before and after repaintTest() and output repaint in layout test result.

In overlay, visualize the minimum repaint in expected results.

In this way, when making repaint change and checking actual results,
we can easily discover under-repaint bugs if we see any solid black
area not covered by repaint rects.

This CL depends on chromium-side CL https://codereview.chromium.org/301243022.

Review URL: https://codereview.chromium.org/315443002

git-svn-id: svn://svn.chromium.org/blink/trunk@176257 bbb929c8-8fbe-4397-9dbb-9b2b20218538