Refactor code that assigns layers to a squashing layer, and that updates
squashing layer geometry, to abstract away the fact that the squashing
layer is positioned relative to m_graphicsLayer a little bit more.



BUG=372837

Review URL: https://codereview.chromium.org/285843002

git-svn-id: svn://svn.chromium.org/blink/trunk@174320 bbb929c8-8fbe-4397-9dbb-9b2b20218538