Simplify RenderLayer::setParent

There's no need call these functions on RenderLayerCompositor.
RenderLayer::addChild and RenderLayer::removeChild already call them.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/261053004

git-svn-id: svn://svn.chromium.org/blink/trunk@173140 bbb929c8-8fbe-4397-9dbb-9b2b20218538