Establish MojoChildConnection from BrowserChildProcessHostImpl

content/public/app/mojo/content_browser_manifest.json

 {
   "manifest_version": 1,
   "name": "exe:content_browser",
   "process-group": "browser",
   "display_name": "Content Browser",
   "capabilities": {
     // Remove this once http://crbug.com/632818 is fixed.
     "provided": {
       "all_interfaces": [ "*" ]
     },
     "required": {
       "*": { "classes": [ "app" ] },
 
       // We allow the browser to connect to any interfaces exposed by its child
       // processes.
+      "exe:content_gpu": { "classes": [ "all_interfaces" ] },
+      "exe:content_plugin": { "classes": [ "all_interfaces" ] },
       "exe:content_renderer": { "classes": [ "all_interfaces" ] },
-      "exe:content_gpu": { "classes": [ "all_interfaces" ] },
       "exe:content_utility": { "classes": [ "all_interfaces" ] },
 
       "mojo:shell": {
         "classes": [
           "shell:client_process",
           "shell:instance_name",
           "shell:user_id"
         ]
       },
       "mojo:file": {
         "interfaces": [
           "leveldb::mojom::LevelDBService",
           "user_service::UserService"
         ]
       },
       "mojo:media": { "classes": [ "all_interfaces" ] }
     }
   }
 }