Factor clip path painting code out of LayerPainter::paintLayerContents into a ClipPathPainter helper class.

BUG=

Review URL: https://codereview.chromium.org/717263002

git-svn-id: svn://svn.chromium.org/blink/trunk@185254 bbb929c8-8fbe-4397-9dbb-9b2b20218538