[CSSRegions] -webkit-background-clip: text; does not clip the background in regions

Force black text behavior if flow thread is in the text clip paintint phase.
(Original WebKit patch by Max Vujovic)

Make style checker pass for modified files.

BUG=307047

Review URL: https://codereview.chromium.org/26931005

git-svn-id: svn://svn.chromium.org/blink/trunk@160433 bbb929c8-8fbe-4397-9dbb-9b2b20218538