Extract a MockExtensionSystem to be used with TestExtensionsBrowserClient.

chrome/browser/extensions/menu_manager_unittest.cc

Index: chrome/browser/extensions/menu_manager_unittest.cc
diff --git a/chrome/browser/extensions/menu_manager_unittest.cc b/chrome/browser/extensions/menu_manager_unittest.cc
index 284f5491481b19a5ae00ca2afb4f5160d870be2f..5efe9dc88e9352316af82bf2ded699859286acc5 100644
--- a/chrome/browser/extensions/menu_manager_unittest.cc
+++ b/chrome/browser/extensions/menu_manager_unittest.cc
@@ -453,6 +453,8 @@ TEST_F(MenuManagerTest, ExtensionUnloadRemovesMenuItems) {
   ASSERT_TRUE(manager_.GetItemById(item2->id()) != NULL);
 }
 
+namespace {
+
 // A mock message service for tests of MenuManager::ExecuteCommand.
 class MockEventRouter : public EventRouter {
  public:
@@ -502,6 +504,8 @@ KeyedService* BuildMockExtensionSystem(content::BrowserContext* profile) {
   return new MockExtensionSystem(static_cast<Profile*>(profile));
 }
 
+}  // namespace
+
 // Tests the RemoveAll functionality.
 TEST_F(MenuManagerTest, RemoveAll) {
   // Try removing all items for an extension id that doesn't have any items.