mash: Fix launching touch hud app in mash mode.

chrome/app/mojo/chrome_manifest.json

 {
   "manifest_version": 1,
   "name": "exe:chrome",
   "display_name": "Chrome",
   "capabilities": {
     "required": {
       "*": { "classes": [ "app" ] },
       "mojo:ash": {
         "interfaces": [
           "ash::mojom::ShelfController",
           "ash::mojom::WallpaperController"
         ]
+      },
+      "mojo:touch_hud": {
+        "interfaces": [
+          "mash::mojom::Launchable"
+        ]
       }
     },
     "provided": {
       "mash:launchable": [ "mash::mojom::Launchable" ],
       "app_list:presenter": [ "app_list::mojom::AppListPresenter" ]
     }
   }
 }