Remove impossible early return from CompositedLayerMapping::updateGraphicsLayerGeometry

This case can never occur. This function is now only called from a full
compositing update, which cleans out all the dirty bits in the z-list. I ran
all the LayoutTests with a CRASH() in this codepath and it never hit.

Review URL: https://codereview.chromium.org/329613002

git-svn-id: svn://svn.chromium.org/blink/trunk@175915 bbb929c8-8fbe-4397-9dbb-9b2b20218538