Move ChromeExtensionHostMsg_AttachGuest to extensions.

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 <string>
 #include <vector>
 
@@ skipping 700 matching lines @@
                     bool /* is_top_level */,
                     std::string /* name */)
 
 // Tells listeners that a detailed message was reported to the console by
 // WebKit.
 IPC_MESSAGE_ROUTED4(ExtensionHostMsg_DetailedConsoleMessageAdded,
                     base::string16 /* message */,
                     base::string16 /* source */,
                     extensions::StackTrace /* stack trace */,
                     int32 /* severity level */)
+
+// Sent by the renderer to set initialization parameters of a Browser Plugin
+// that is identified by |element_instance_id|.
+IPC_MESSAGE_CONTROL4(ExtensionHostMsg_AttachGuest,
+                     int /* routing_id */,
+                     int /* element_instance_id */,
+                     int /* guest_instance_id */,
+                     base::DictionaryValue /* attach_params */)