First pass at experimental omnibox API. There are plenty of rough edges and

chrome/browser/extensions/extension_message_service.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/extensions/extension_message_service.h"
 
 #include "base/json/json_writer.h"
 #include "base/singleton.h"
 #include "base/stl_util-inl.h"
 #include "base/values.h"
@@ skipping 159 matching lines @@
   DCHECK_EQ(listeners_[event_name].count(render_process_id), 1u)
       << " PID=" << render_process_id << " event=" << event_name;
   listeners_[event_name].erase(render_process_id);
 
   if (extension_devtools_manager_.get()) {
     extension_devtools_manager_->RemoveEventListener(event_name,
                                                      render_process_id);
   }
 }
 
+bool ExtensionMessageService::HasEventListener(
+    const std::string& event_name) {
+  return (listeners_.find(event_name) != listeners_.end() &&
+          !listeners_[event_name].empty());
+}
+
 void ExtensionMessageService::AllocatePortIdPair(int* port1, int* port2) {
   AutoLock lock(next_port_id_lock_);
 
   // TODO(mpcomplete): what happens when this wraps?
   int port1_id = next_port_id_++;
   int port2_id = next_port_id_++;
 
   DCHECK(IS_OPENER_PORT_ID(port1_id));
   DCHECK(GET_OPPOSITE_PORT_ID(port1_id) == port2_id);
   DCHECK(GET_OPPOSITE_PORT_ID(port2_id) == port1_id);
@@ skipping 354 matching lines @@
 
     if (current->second->opener.sender == sender) {
       CloseChannelImpl(current, GET_CHANNEL_OPENER_ID(current->first),
                        notify_other_port);
     } else if (current->second->receiver.sender == sender) {
       CloseChannelImpl(current, GET_CHANNEL_RECEIVERS_ID(current->first),
                        notify_other_port);
     }
   }
 }