services/shell: Turn on sandboxing for mojo apps.

services/ui/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:ui",
   "display_name": "UI Service",
   "capabilities": {
     "provided": {
       // A collection of interfaces needed by a generic client of mus.
       // Additional interfaces may be requested a-la-carte.
       "app": [
         "ui::mojom::Clipboard",
         "ui::mojom::DisplayManager",
         "ui::mojom::Gpu",
         "ui::mojom::GpuService",
         "ui::mojom::IMEServer",
         "ui::mojom::InputDeviceServer",
         "ui::mojom::WindowTreeFactory"
       ],
       // Interfaces provided by mus-gpu for mus-ws.
       "ozone": [
         "ui::ozone::mojom::DeviceCursor"
       ],
       "test": [
         "ui::mojom::WindowServerTest"
       ]
     },
     "required": {
       "mojo:ui": { "classes": [ "ozone" ] },
       "*": { "classes": [ "app" ] },
-      "mojo:shell": { "classes": [ "shell:all_users", "shell:explicit_class" ] }
+      "mojo:shell": {
+        "classes": [
+          "shell:all_users",
+          "shell:explicit_class",
+          "shell:unsandboxed"
+        ]
+      }
     }
   }
 }