Browser+LibWeb+WebContent: Make the "Debug" menu work in multi-process

This patch adds an IPC call for debugging requests. It's stringly typed
and very simple, and allows us to easily implement all the features in
the Browser's Debug menu.

Userland/Services/WebContent/WebContentServer.ipc

@@ -18,4 +18,6 @@ endpoint WebContentServer = 89
     MouseUp(Gfx::IntPoint position, unsigned button, unsigned buttons, unsigned modifiers) =|
 
     KeyDown(i32 key, unsigned modifiers, u32 code_point) =|
+
+    DebugRequest(String request, String argument) =|
 }