Fix clippingContainer for fixed position elements.

Fixed position elements are clipped by CSS clip ancestors,
which may be between the fixed position element and its
containingBlock. So clippingAncestor needs to walk up the
ancestors of a fixed positioned element manually instead
of using containingBlock directly.

I believe this only applies to fixed position elements.
Other elements can use containingBlock because an absolutely
positioned ancestor will never get skipped in the
containingBlock traversal and CSS clip only applies to
absolutely positioned elements.

BUG=347172

Review URL: https://codereview.chromium.org/179833006

git-svn-id: svn://svn.chromium.org/blink/trunk@168473 bbb929c8-8fbe-4397-9dbb-9b2b20218538