Dispel the myth that enclosingLayer can return zero

A bunch of code was needlessly null-checking enclosingLayer. That function can
only return zero for RenderObjects that are outside of the render tree.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/215843002

git-svn-id: svn://svn.chromium.org/blink/trunk@170233 bbb929c8-8fbe-4397-9dbb-9b2b20218538