Fix bugs in clip parent calculations

I noticed two bugs in the clip parent code - when walking up the layers
in the stacking tree looking for clipping layers, I'd forgotten to
consider the layers themselves (I'd only considered their clipping
containers). I'd also incorrectly gated promotion for having a clip
parent on having an ancestor scrolling layer that needed composited
scrolling. This condition was bogus: we often need a clip parent
precisely because we have no ancestor scrolling layer (eg, we're fixed
positioned and have 'escaped' our ancestor scrollers in tree order).

BUG=None

Review URL: https://codereview.chromium.org/402283003

git-svn-id: svn://svn.chromium.org/blink/trunk@178649 bbb929c8-8fbe-4397-9dbb-9b2b20218538