Extensions installed by policy overrun previously installed extensions.

chrome/browser/extensions/pending_extension_manager.h

 // Copyright (c) 2011 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_PENDING_EXTENSION_MANAGER_H_
 #define CHROME_BROWSER_EXTENSIONS_PENDING_EXTENSION_MANAGER_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 54 matching lines @@
   // TODO(akalin): Replace |install_silently| with a list of
   // pre-enabled permissions.
   bool AddFromSync(
       const std::string& id,
       const GURL& update_url,
       PendingExtensionInfo::ShouldAllowInstallPredicate should_allow_install,
       bool install_silently);
 
   // Given an extension id and an update URL, schedule the extension
   // to be fetched, installed, and activated.
+  // If |overwrite| is true, an existing extension with the same id will
+  // be replaced by the extension fetched.
   void AddFromExternalUpdateUrl(const std::string& id,
                                 const GURL& update_url,
-                                Extension::Location location);
+                                Extension::Location location,
+                                bool overwrite);
 
   // Add a pending extension record for an external CRX file.
   void AddFromExternalFile(
       const std::string& id,
       Extension::Location location);
 
  private:
   // Assumes an extension with id |id| is not already installed.
   // Return true if the extension was added.
   bool AddExtensionImpl(
@@ skipping 23 matching lines @@
                            UpdatePendingExtensionAlreadyInstalled);
   friend class ExtensionUpdaterTest;
   friend void SetupPendingExtensionManagerForTest(
       int count, const GURL& update_url,
       PendingExtensionManager* pending_extension_manager);
 
   DISALLOW_COPY_AND_ASSIGN(PendingExtensionManager);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_PENDING_EXTENSION_MANAGER_H_