Revert of Remove RenderLayer::collectFragments. (patchset #1 id:1 of https://codereview.chromium.org/778043005/)

Reason for revert:
This broke hit testing.  Hit testing always returns the root node now. :)

Original issue's description:
> 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
>
> Committed: https://chromium.googlesource.com/external/mojo/+/4a3b676dc3387b64575d4a407c90fa8a6f466f4c

TBR=esprehn@chromium.org,rafaelw@chromium.org,abarth@chromium.org,ojan@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/788383002