Restore GC state after painting with a "reference" [url(#foo)] clip-path

If the referenced 'clipPath' is a simple (single shape), the GC state is
mutated with the clip-path shape. It's however not guaranteed that the
GC state is restored - and hence the clip removed - before painting later
content.
Make sure the state is restored at the exit of
RenderLayer::paintLayerContents by using a GCSS. Convert the
'clip-path: <shape>'-case to use the GCSS as well.

BUG=391291

Review URL: https://codereview.chromium.org/365293002

git-svn-id: svn://svn.chromium.org/blink/trunk@177586 bbb929c8-8fbe-4397-9dbb-9b2b20218538