Properly clip the view background with slimming paint

We sadly break out of our renderer bounds when drawing the document
renderer's background. Avoid improperly clipping in that case.

R=chrishtr@chromium.org,pdr@chromium.org

Review URL: https://codereview.chromium.org/650233004

git-svn-id: svn://svn.chromium.org/blink/trunk@184019 bbb929c8-8fbe-4397-9dbb-9b2b20218538