Fix a bogus assertion in CLM

In the CompositedLayerMapping's doPaintTask function we ASSERT that
we never get here if we paint into our composited ancestor (i.e., we
don't paint ourself). This doesn't make sense for the squashing layer.
Even if the layer that owns the backing happens to paint into its
ancestor, the squashed layers may well paint into the squashing layer.

BUG=394413

Review URL: https://codereview.chromium.org/476583002

git-svn-id: svn://svn.chromium.org/blink/trunk@180328 bbb929c8-8fbe-4397-9dbb-9b2b20218538