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.
