Bug 959118 - Dump layer tree with layer scope on the viewer. r=dglastonbury

We also want to dump layer tree on the viewer, so we
can check the layer tree and layerscope together
in the viewer. This can help us resolve more gfx bugs.

In this patch, I only add a part of the layer data to
the protocol buffer packet, and you can check the
.proto file for more information if you want to add
more layer data.

By the way, as Jeff's suggestion, use auto & MakeUnique<>()
to make the UniquePtr initialization more concise.