[Extensions] Begin making Extension port initialization asynchronous

extensions/common/extension_messages.h

 // 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.
 
 // IPC messages for extensions.
 // Multiply-included message file, hence no include guard.
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 656 matching lines @@
 IPC_MESSAGE_CONTROL4(ExtensionHostMsg_RemoveFilteredListener,
                      std::string /* extension_id */,
                      std::string /* name */,
                      base::DictionaryValue /* filter */,
                      bool /* lazy */)
 
 // Notify the browser that an event has finished being dispatched.
 IPC_MESSAGE_ROUTED1(ExtensionHostMsg_EventAck, int /* message_id */)
 
 // Open a channel to all listening contexts owned by the extension with
-// the given ID.  This always returns a valid port ID which can be used for
-// sending messages.  If an error occurred, the opener will be notified
-// asynchronously.
-IPC_SYNC_MESSAGE_CONTROL4_1(ExtensionHostMsg_OpenChannelToExtension,
-                            int /* frame_routing_id */,
-                            ExtensionMsg_ExternalConnectionInfo,
-                            std::string /* channel_name */,
-                            bool /* include_tls_channel_id */,
-                            int /* port_id */)
+// the given ID. This responds asynchronously with ExtensionMsg_AssignPortId.
+// If an error occurred, the opener will be notified asynchronously.
+IPC_MESSAGE_CONTROL5(ExtensionHostMsg_OpenChannelToExtension,
+                     int /* frame_routing_id */,
+                     ExtensionMsg_ExternalConnectionInfo,
+                     std::string /* channel_name */,
+                     bool /* include_tls_channel_id */,
+                     int /* request_id */)
+
+// The response to a request to open an extension message port, including the
+// global port id and the request id.
+IPC_MESSAGE_ROUTED2(ExtensionMsg_AssignPortId,
+                    int /*port_id */,
+                    int /* request_id */)
 
 IPC_SYNC_MESSAGE_CONTROL2_1(ExtensionHostMsg_OpenChannelToNativeApp,
                             int /* frame_routing_id */,
                             std::string /* native_app_name */,
                             int /* port_id */)
 
 // Get a port handle to the given tab.  The handle can be used for sending
 // messages to the extension.
 IPC_SYNC_MESSAGE_CONTROL4_1(ExtensionHostMsg_OpenChannelToTab,
                             int /* frame_routing_id */,
@@ skipping 174 matching lines @@
 
 // The browser sends this message in response to all service worker extension
 // api calls. The response data (if any) is one of the base::Value subclasses,
 // wrapped as the first element in a ListValue.
 IPC_MESSAGE_CONTROL5(ExtensionMsg_ResponseWorker,
                      int /* thread_id */,
                      int /* request_id */,
                      bool /* success */,
                      base::ListValue /* response wrapper (see comment above) */,
                      std::string /* error */)