Break updateSquashingLayerGeometry out of updateGraphicsLayerGeometry

A big block of code which probably works better as a helper function.

BUG=338896

Review URL: https://codereview.chromium.org/149393008

git-svn-id: svn://svn.chromium.org/blink/trunk@166097 bbb929c8-8fbe-4397-9dbb-9b2b20218538