Fix squashing layers with different clipping containers.

Layers may not squash unless they share the same clipping
container as the squashing CLM owner. Previously the
implementation of this had two bugs: (a) it traversed in
paint order, not tree order, and kept a running clipping
ancestor along the way, and (b) it did not use the same
logic as the CompositedLayerMapping did for allocating an
m_ancestorClippingLayer for a squashing CLM.

BUG=358507

Review URL: https://codereview.chromium.org/222863002

git-svn-id: svn://svn.chromium.org/blink/trunk@170776 bbb929c8-8fbe-4397-9dbb-9b2b20218538