Remove unused create_layer_group_for_pipeline
Don't reuse an old pipeline for a new root layer

When navigating to a new URL, we need to make sure compositor messages go to
the new page's pipeline, not the previous one.
auto merge of #2084 : mbrubeck/servo/navigate-new-pipeline-fix, r=larsbergstrom

After clicking a link to load a new page, the new page content is never painted.  This is because the compositor is still sending render messages to old frame tree's pipeline, not the new one.

This patch fixes this by ensuring that the compositor's new root layer is attached to the correct pipeline. (Note: You will also need to apply #2080 if you want to test this without crashing.)