Don't re-use parent clip for composited descendants in PaintInvalidationState

Ultimately, the compositor will correctly apply clipping of composited
content. PaintInvalidationState's clip incorrectly re-used the parents'
coordinate system to clip composited descendants.

BUG=395040

Review URL: https://codereview.chromium.org/404823006

git-svn-id: svn://svn.chromium.org/blink/trunk@178684 bbb929c8-8fbe-4397-9dbb-9b2b20218538