Refactor CompositedLayerMapping::updateGraphicsLayerGeometry into shorter methods, one for each graphics layer.

BUG=372837

Review URL: https://codereview.chromium.org/298723009

git-svn-id: svn://svn.chromium.org/blink/trunk@174639 bbb929c8-8fbe-4397-9dbb-9b2b20218538