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

chrome/browser/extensions/extension_message_service.h

Index: chrome/browser/extensions/extension_message_service.h
diff --git a/chrome/browser/extensions/extension_message_service.h b/chrome/browser/extensions/extension_message_service.h
index c205bb91392e4d30cd8da51875b5998c50e983cd..bf499482f8d7f6a63020e21d8ea871887d17558b 100644
--- a/chrome/browser/extensions/extension_message_service.h
+++ b/chrome/browser/extensions/extension_message_service.h
@@ -74,6 +74,9 @@ class ExtensionMessageService
   void RemoveEventListener(const std::string& event_name,
                            int render_process_id);
 
+  // Returns true if there is at least one listener for the given event.
+  bool HasEventListener(const std::string& event_name);
+
   // Closes the message channel associated with the given port, and notifies
   // the other side.
   void CloseChannel(int port_id);