When invalidating an area for a background image on the document, invalidate the larger of view() and documentElement().

This is required per spec.

BUG=419660

Review URL: https://codereview.chromium.org/645643002

git-svn-id: svn://svn.chromium.org/blink/trunk@183514 bbb929c8-8fbe-4397-9dbb-9b2b20218538