Fix slimming paint clipping and add a debug fill color

Fix slimming paint clipping and add a debug fill color

The Slimming Paint codepath wasn't properly adding paint offsets
to the applied clip rects, causing only a fraction of text to be
drawn. Also adding a debug red background to make it more obvious
when we're failing to paint. Cribbed from FrameView's old paint
code.

BUG=423071
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183634

[leviw_travelin_and_unemployed, 2014-10-13 22:34:41]

leviw@chromium.org changed reviewers:
+ chrishtr@chromium.org, pdr@chromium.org

[chrishtr, 2014-10-13 22:42:52]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2014-10-13 22:42:53]

lgtm

[commit-bot: I haz the power, 2014-10-13 22:43:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/654713003/1

[commit-bot: I haz the power, 2014-10-14 00:59:09]

Committed patchset #1 (id:1) as 183634