Teach the walker to vend a layout inspector actor and serialize node
display types.
The layout inspector now returns Firefox-compatible grid actor forms
for getGrids and getCurrentGrid. It also supports Firefox's fallback
walker.getNodeFromActor(grid, ["containerEl"]) path, which the grid
panel uses when a grid form does not already contain a container node
actor ID.
This is to prepare for an upcoming change where we will need to track
replies to messages by ID. We will be able to add parameters to this
structure without having to edit every single actor subclass header
file.
The DevTools client will ask for this actor before trying to render any
box model or computed style information. We can just stub out this actor
for now.
