Update scroll parent correctly for squashing layers.

Currently, we only set a scroll parent for 
"localRootForOwningLayer".I.e., the highest
GraphicsLayer in the CLM-managed hierarchy, excluding
the squashing containment layer. But for "scroll
buddies", we need the scroll child to be the very top
of the hierarchy.

We also need to ensure that scroll buddies are not
prevented from squashing together because of the
additional "I've got a scroll parent" compositing
reason.

This cl also disables chicken-egg asserts that trip
w/ universal accelerated overflow scroll and changes
how we compute composited bounds for squashed layers
(we currently pull the bounds from the clipper, but
it picks the wrong "clipping root" in this case).

With this CL, universal accelerated overflow scrolling
and squashing work nearly perfectly modulo known hit
testing bugs (which can prevent touch scrolling) and
repaint issues (separate issue reported here
https://code.google.com/p/chromium/issues/detail?id=351790).

BUG=328156

Review URL: https://codereview.chromium.org/131543013

git-svn-id: svn://svn.chromium.org/blink/trunk@169776 bbb929c8-8fbe-4397-9dbb-9b2b20218538