Make browser process a singleton service

ash/mus/manifest.json

 {
   "name": "ash",
   "display_name": "Ash Window Manager and Shell",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         // Modifications here should correspond with changes to
         // chrome_content_browser_manifest_overlay.json.
         "ash": [
           "app_list::mojom::AppList",
           "ash::mojom::AcceleratorController",
           "ash::mojom::CastConfig",
           "ash::mojom::LocaleNotificationController",
           "ash::mojom::MediaController",
           "ash::mojom::NewWindowController",
           "ash::mojom::SessionController",
           "ash::mojom::ShelfController",
           "ash::mojom::ShutdownController",
           "ash::mojom::SystemTray",
           "ash::mojom::TouchViewManager",
           "ash::mojom::VpnList",
           "ash::mojom::WallpaperController"
         ],
         "mus:window_manager": [ "ui::mojom::AcceleratorRegistrar" ]
       },
       "requires": {
         "*": [ "accessibility", "app" ],
-        "content_browser": [ "ash" ],
         "preferences": [ "preferences_manager" ],
         "ui": [ "display_test", "window_manager" ],
         "touch_hud": [ "mash:launchable" ]
       }
     }
   }
 }