Extensions installed by policy overrun previously installed extensions.

chrome/browser/extensions/pending_extension_manager.cc

 // 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.
 
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/pending_extension_manager.h"
 #include "chrome/common/extensions/extension.h"
 #include "content/browser/browser_thread.h"
@@ skipping 56 matching lines @@
 }
 
 void PendingExtensionManager::AddFromExternalUpdateUrl(
     const std::string& id, const GURL& update_url,
     Extension::Location location) {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   const bool kIsFromSync = false;
   const bool kInstallSilently = true;
 
-  if (service_.IsExternalExtensionUninstalled(id))
-    return;
-
-  if (service_.GetInstalledExtension(id)) {
-    LOG(DFATAL) << "Trying to add extension " << id
-                << " by external update, but it is already installed.";
-    return;
+  const Extension* extension = service_.GetInstalledExtension(id);
+  if (extension &&
+      location == Extension::GetHigherPriorityLocation(location,
+                                                       extension->location())) {
+    // If the new location has higher priority than the location of an existing
+    // extension, let the update process overwrite the existing extension.
+  } else {
+    if (service_.IsExternalExtensionUninstalled(id)) {
+      return;
+    }
+    if (extension) {
+      LOG(DFATAL) << "Trying to add extension " << id
+                  << " by external update, but it is already installed.";
+      return;
+    }
   }
 
   AddExtensionImpl(id, update_url, &AlwaysInstall,
                    kIsFromSync, kInstallSilently,
                    location);
 }
 
 
 void PendingExtensionManager::AddFromExternalFile(
     const std::string& id,
@@ skipping 55 matching lines @@
     return true;
   }
   return false;
 }
 
 void PendingExtensionManager::AddForTesting(
     const std::string& id,
     const PendingExtensionInfo& pending_extension_info) {
   pending_extension_map_[id] = pending_extension_info;
 }