With CompositorState in place, Browser and WebContent need an actual
wire protocol to drive it: which calls are sync, who validates that a
WebContent is allowed to touch a given context, how compositor loss
propagates back to the renderer. This commit fills in the four endpoints
around CompositorState and the matching WebContent-side helpers so the
future remote host can forward CompositorHost calls directly. The
service still has no clients, so default rendering is unchanged.
The compositor is moving into a dedicated helper process. That requires
a process to launch, channels for Browser and WebContent to talk to it
over, and client proxies on each side. Land all of that as an inert
scaffold first, gated behind --enable-compositor-process, so the default
rendering path is unchanged and later commits can fill in the protocol,
the service-side state, and the runtime switch against a stable target.
