Overlay expected and actual repaint rects for LayoutTests.

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.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175182

[Xianzhu, 2014-05-30 00:25:44]

A known issue of this patch: it can't handle layers with offset and
transformations yet.

[dsinclair, 2014-05-30 01:06:17]

non-owner lgtm

[esprehn, 2014-05-30 02:02:30]

dpranke@ or ojan@ are your best reviewers for python code.

[Dirk Pranke, 2014-05-30 19:00:50]

lgtm. thanks for working on this.

[Xianzhu, 2014-05-30 19:18:35]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-05-30 19:19:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/303223008/70001

[Xianzhu, 2014-05-30 21:42:49]

The CQ bit was unchecked by wangxianzhu@chromium.org

[Xianzhu, 2014-05-30 23:11:50]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-05-30 23:13:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/303223008/90001

[commit-bot: I haz the power, 2014-05-31 01:14:45]

Change committed as 175182