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

chrome/browser/extensions/extensions_service.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSIONS_SERVICE_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSIONS_SERVICE_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 245 matching lines @@
   void ClearProvidersForTesting();
 
   // Sets an ExternalExtensionProvider for the service to use during testing.
   // |location| specifies what type of provider should be added.
   void SetProviderForTesting(Extension::Location location,
                              ExternalExtensionProvider* test_provider);
 
   // Called when the initial extensions load has completed.
   virtual void OnLoadedInstalledExtensions();
 
-  // Called when an extension has been loaded.
-  void OnExtensionLoaded(Extension* extension,
+  // Called when an extension has been loaded. Returns false if the extension
+  // failed to load (for example, if it has already been loaded).
+  bool OnExtensionLoaded(Extension* extension,
                          bool allow_privilege_increase);
 
   // Called by the backend when an extension has been installed.
   void OnExtensionInstalled(Extension* extension,
                             bool allow_privilege_increase);
 
   // Called by the backend when an external extension is found.
   void OnExternalExtensionFound(const std::string& id,
                                 const std::string& version,
                                 const FilePath& path,
@@ skipping 240 matching lines @@
 
   // A map of all external extension providers.
   typedef std::map<Extension::Location,
                    linked_ptr<ExternalExtensionProvider> > ProviderMap;
   ProviderMap external_extension_providers_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionsServiceBackend);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSIONS_SERVICE_H_