Make it possible to have multiple InspectorBackends

sky/services/inspector/inspector.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module sky;
 
+interface InspectorServer {
+  Listen(int32 port) => ();
+};
+
 [Client=InspectorBackend]
 interface InspectorFrontend {
-  Listen(int32 port);
   SendMessage(string message);
 };
 
 [Client=InspectorFrontend]
 interface InspectorBackend {
   OnConnect();
   OnDisconnect();
   OnMessage(string message);
 };