Fix three nits related to layer positions

This CL makes this minor changes to cleanup and improve the documentation of
this code. This CL shouldn't have any effect on behavior.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/449663002

git-svn-id: svn://svn.chromium.org/blink/trunk@179665 bbb929c8-8fbe-4397-9dbb-9b2b20218538