Add a function triggering extension installed to ExtensionRegistryObserver

extensions/browser/extension_registry_observer.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.
 
 #ifndef EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 #define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_
 
 #include "extensions/common/extension.h"
 
 namespace content {
@@ skipping 15 matching lines @@
   // in the enabled_extensions set of ExtensionRegistry.
   virtual void OnExtensionLoaded(
       content::BrowserContext* browser_context,
       const Extension* extension) {}
 
   // Called after an extension is unloaded. The extension no longer exists in
   // any of the ExtensionRegistry sets (enabled, disabled, etc.).
   virtual void OnExtensionUnloaded(content::BrowserContext* browser_context,
                                    const Extension* extension,
                                    UnloadedExtensionInfo::Reason reason) {}
+
+  // Called when |extension| is about to be installed. |is_update| is true if
+  // the installation is the result of it updating, in which case |old_name| is
+  // the name of the extension's previous version.
+  // The ExtensionRegistry will not be tracking |extension| at the time this
+  // event is fired, but will be immediately afterwards (note: not necessarily
+  // enabled; it might be installed in the disabled or even blacklisted sets,
+  // for example).
+  // Note that it's much more common to care about extensions being loaded
+  // (OnExtensionLoaded).
+  virtual void OnExtensionWillBeInstalled(
+      content::BrowserContext* browser_context,
+      const Extension* extension,
+      bool is_update,
+      const std::string& old_name) {}
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_OBSERVER_H_