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

Review URL: https://codereview.chromium.org/654713003

git-svn-id: svn://svn.chromium.org/blink/trunk@183634 bbb929c8-8fbe-4397-9dbb-9b2b20218538