Adds associated interface support to IPC::Channel

ipc/ipc.mojom

 // Copyright 2016 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 IPC.mojom;
 
 struct SerializedHandle {
   handle the_handle;
 
   enum Type {
     MOJO_HANDLE,
     PLATFORM_FILE,
     WIN_HANDLE,
     MACH_PORT,
   };
 
   Type type;
 };
 
+// A placeholder interface type since we don't yet support generic associated
+// message pipe handles.
+interface GenericInterface {};
+
 interface Channel {
   Receive(array<uint8> data, array<SerializedHandle>? handles);
+
+  GetAssociatedInterface(string name, associated GenericInterface& request);
 };
 
 // An interface for connecting a pair of Channel interfaces representing a
 // bidirectional IPC channel.
 interface Bootstrap {
   // Initializes a Chrome IPC channel over |to_client_channel| and
   // |to_server_channel|. Each side also sends its PID to the other side.
   Init(associated Channel& to_client_channel,
        associated Channel to_server_channel,
        int32 pid) => (int32 pid);
 };