Backing owner determines scrolling mechanism.

We had previously set a CLM's scroll parent to be either the scroll
parent it determined for itself, or the scroll parent of its squashed
layers. I believe that this was to support the case where you could
squash layers that scrolled wrt each other, but this still seems
suspicious.

It's very possible that the layer that owns the backing (the layer
being squashed into), is a descendant of the scroller, and will
scroll with it naturally, with no need for a special scroll parent
pointer. We take special care not to squash together layers that scroll
with respect to each other now, so if any of the squashees would have
previously required a scroll parent, they need not have one if the
squashing target doesn't: however the squashing target manages to
scroll around, in the same way the squashees will get taken along for
the ride. They do not need to influence the scrolling machinery.

R=hartmanng@chromium.org
BUG=365773

Review URL: https://codereview.chromium.org/249613003

git-svn-id: svn://svn.chromium.org/blink/trunk@172421 bbb929c8-8fbe-4397-9dbb-9b2b20218538