Only send events to renderers that are listening to an event....

chrome/common/render_messages_internal.h

 // Copyright (c) 2009 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.
 
 // This header is meant to be included in multiple passes, hence no traditional
 // header guard.
 // See ipc_message_macros.h for explanation of the macros and passes.
 
 #include <string>
 #include <vector>
@@ skipping 1278 matching lines @@
                       double /* left_channel */,
                       double /* right_channel */)
 
   // A renderer sends this message when an extension process starts an API
   // request. If callback id is -1, no response will be sent.
   IPC_MESSAGE_ROUTED3(ViewHostMsg_ExtensionRequest,
                       std::string /* name */,
                       std::string /* argument */,
                       int /* callback id */)
 
+  // Notify the browser that this renderer added a listener to an event.
+  IPC_MESSAGE_CONTROL1(ViewHostMsg_ExtensionAddListener,
+                       std::string /* name */)
+
+  // Notify the browser that this renderer removed a listener from an event.
+  IPC_MESSAGE_CONTROL1(ViewHostMsg_ExtensionRemoveListener,
+                       std::string /* name */)
+
 #if defined(OS_MACOSX)
   // On OSX, we cannot allocated shared memory from within the sandbox, so
   // this call exists for the renderer to ask the browser to allocate memory
   // on its behalf. We return a file descriptor to the POSIX shared memory.
   IPC_SYNC_MESSAGE_CONTROL1_1(ViewHostMsg_AllocTransportDIB,
                               size_t, /* bytes requested */
                               TransportDIB::Handle /* DIB */)
 
   // Since the browser keeps handles to the allocated transport DIBs, this
   // message is sent to tell the browser that it may release them when the
@@ skipping 30 matching lines @@
   // by ViewHostMsg_OpenChannelToExtension.
   IPC_MESSAGE_CONTROL2(ViewHostMsg_ExtensionPostMessage,
                        int /* port_id */,
                        std::string /* message */)
 
   // Message to show a popup menu using native cocoa controls (Mac only).
   IPC_MESSAGE_ROUTED1(ViewHostMsg_ShowPopup,
                       ViewHostMsg_ShowPopup_Params)
 
 IPC_END_MESSAGES(ViewHost)