Simplify CompositedLayerMapping's hasVisibleNonCompositingDescendant

This CL moves the early out to the top of the function, which lets us merge the
two stacking node iterations.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/183973017

git-svn-id: svn://svn.chromium.org/blink/trunk@168360 bbb929c8-8fbe-4397-9dbb-9b2b20218538