Fix new -Wnull-conversion violations detected by a newer clang.

chrome/browser/extensions/test_extension_service.cc

 // Copyright (c) 2012 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 "chrome/browser/extensions/crx_installer.h"
 #include "chrome/browser/extensions/extension_sync_data.h"
 #include "chrome/browser/extensions/test_extension_service.h"
 #include "sync/api/sync_error_factory.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 13 matching lines @@
   ADD_FAILURE();
   return NULL;
 }
 
 bool TestExtensionService::UpdateExtension(
     const std::string& id,
     const FilePath& path,
     const GURL& download_url,
     CrxInstaller** out_crx_installer) {
   ADD_FAILURE();
-  return NULL;
+  return false;
 }
 
 const Extension* TestExtensionService::GetExtensionById(
     const std::string& id, bool include_disabled) const {
   ADD_FAILURE();
   return NULL;
 }
 
 const Extension* TestExtensionService::GetInstalledExtension(
     const std::string& id) const {
@@ skipping 64 matching lines @@
 void TestExtensionService::UnloadExtension(
     const std::string& extension_id,
     extension_misc::UnloadedExtensionReason reason) {
   ADD_FAILURE();
 }
 
 void TestExtensionService::SyncExtensionChangeIfNeeded(
     const Extension& extension) {
   ADD_FAILURE();
 }