Over to the compositor side:
* The message is received and this comes in as a call to PLayerTransactionParent::AllocPGrallocBufferParentAllocPGrallocBuffer, implemented in LayerTransactionParent.cpp.
* This calls GrallocBufferActor::Create(...), which actually creates the GraphicBuffer* and a GrallocBufferActor* (The GrallocBufferActor contains a sp<GraphicBuffer> that references the newly-created GraphicBuffer*).
* GrallocBufferActor::Create returns the GrallocBufferActor as a PGrallocBufferParent*, and the GraphicBuffer* as a MaybeMagicGrallocBufferHandle.