Remove CompositedLayerMapping::compositingLayerType()

Nothing calls this function which is just "informative" anyway. We should
remove it.

Review URL: https://codereview.chromium.org/181043017

git-svn-id: svn://svn.chromium.org/blink/trunk@168114 bbb929c8-8fbe-4397-9dbb-9b2b20218538