Hosting the discardable memory service in the mus process

content/public/app/mojo/content_plugin_manifest.json

 {
   "name": "content_plugin",
   "display_name": "Content (plugin process)",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "browser": [ "IPC::mojom::ChannelBootstrap" ],
         "service_manager:service_factory": [
           "service_manager::mojom::ServiceFactory"
         ]
       },
       "requires": {
-        "content_browser": [ "plugin" ]
+        "content_browser": [ "plugin" ],
+        "ui": [ "discardable_memory" ]
       }
     }
   }
 }