Bug 863324 - Rename PLayers into PLayerTransaction, r=jrmuizel

--HG--
rename : gfx/layers/ipc/ShadowLayersChild.cpp => gfx/layers/ipc/LayerTransactionChild.cpp
rename : gfx/layers/ipc/ShadowLayersChild.h => gfx/layers/ipc/LayerTransactionChild.h
rename : gfx/layers/ipc/ShadowLayersParent.cpp => gfx/layers/ipc/LayerTransactionParent.cpp
rename : gfx/layers/ipc/ShadowLayersParent.h => gfx/layers/ipc/LayerTransactionParent.h
rename : gfx/layers/ipc/PLayers.ipdl => gfx/layers/ipc/PLayerTransaction.ipdl