More refactoring of CLM::updateGraphicsLayerGeometry.

Move m_graphicsLayer config into its own method, adjust squashing layer
update to match style of the rest.
BUG=372837

Review URL: https://codereview.chromium.org/290173013

git-svn-id: svn://svn.chromium.org/blink/trunk@174651 bbb929c8-8fbe-4397-9dbb-9b2b20218538