Fix missing repaints when squashed layers change the GraphicsLayer they paint into.

When a CompositedLayerMapping (CLM) gets destroyed, and it had a
squashingLayer, then we need to issue a repaint to the enclosing
layer that is likely to receive the squashed layer's contents.

No new test needed, covered by existing layout test that just
needs the new baseline.

BUG=333012

Review URL: https://codereview.chromium.org/130273004

git-svn-id: svn://svn.chromium.org/blink/trunk@165182 bbb929c8-8fbe-4397-9dbb-9b2b20218538