Bug 1020169 - Part 1. Remove static constructors. r=mattwoodrow
Bug 1020169 - Part 2. Register nested LayerTree. r=mattwoodrow

Use PCompositor::NotifyChildCreated which was ill-defined for
cross-process case.

Assume that for all LayerTrees managed by a CrossProcessCompositorParent
all share the same CompositorParent.