Revert "Move ExtensionCache to //extensions"

extensions/browser/updater/null_extension_cache.cc

-// 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.
-
-#include "extensions/browser/updater/null_extension_cache.h"
-
-#include "base/callback.h"
-
-namespace extensions {
-
-NullExtensionCache::NullExtensionCache() {
-}
-
-NullExtensionCache::~NullExtensionCache() {
-}
-
-void NullExtensionCache::Start(const base::Closure& callback) {
-  callback.Run();
-}
-
-void NullExtensionCache::Shutdown(const base::Closure& callback) {
-  callback.Run();
-}
-
-void NullExtensionCache::AllowCaching(const std::string& id) {
-}
-
-bool NullExtensionCache::GetExtension(const std::string& id,
-                                      base::FilePath* file_path,
-                                      std::string* version) {
-  return false;
-}
-
-void NullExtensionCache::PutExtension(const std::string& id,
-                                      const base::FilePath& file_path,
-                                      const std::string& version,
-                                      const PutExtensionCallback& callback) {
-  callback.Run(file_path, true);
-}
-
-}  // namespace extensions