Revert of Remove RenderLayer::collectFragments.

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

Committed: https://chromium.googlesource.com/external/mojo/+/7ed3e0f5a37ae784e7d5fde52361fc8484d95be9

[eseidel, 2014-12-10 22:42:25]

Created Revert of Remove RenderLayer::collectFragments.

[commit-bot: I haz the power, 2014-12-10 22:42:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-12-10 22:42:45]

Commit queue rejected this change because it did not recognize the base URL.
Please commit your change manually.

[eseidel, 2014-12-10 22:58:52]

Committed patchset #1 (id:1) manually as
7ed3e0f5a37ae784e7d5fde52361fc8484d95be9 (presubmit successful).