Remove RenderLayer::collectFragments.

Sky always has exactly one per RenderLayer. This patch
gets rid of the hitTestLayer use of LayerFragment
and gets paintLayerContents to create the list of
fragments itself since it's the only caller.

Also, delete dead code from LayerFragment.h.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/778043005