Firefox's grid inspector builds its subgrid tree from two protocol
signals: isSubgrid on each GridActor form, and
Walker.getParentGridNode() for each subgrid container. Teach the walker
to answer the parent grid lookup so Firefox can attach subgrids below
their parent grid in the Layout panel.
Create typed highlighter actors for Firefox's highlighter requests.
The actor keeps the requested type so CssGridHighlighter can use grid
highlighting while the other highlighter types keep the existing box
model-compatible behavior.
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.
Use a fake delegate to cover root, target, inspector, styles, network,
navigation, and accessibility behavior. This is not in-depth, but should
be enough to catch regressions.
