Extract LazyBackgroundTaskQueue from ExtensionSystem.

extensions/browser/mock_extension_system.cc

Index: extensions/browser/mock_extension_system.cc
diff --git a/extensions/browser/mock_extension_system.cc b/extensions/browser/mock_extension_system.cc
index b6f5f8bc10acba93f0ae5dbe8649fbf1b5fbda8e..a9d866a9a599536a60c549aa94b93fe786cc16eb 100644
--- a/extensions/browser/mock_extension_system.cc
+++ b/extensions/browser/mock_extension_system.cc
@@ -51,10 +51,6 @@ InfoMap* MockExtensionSystem::info_map() {
   return nullptr;
 }
 
-LazyBackgroundTaskQueue* MockExtensionSystem::lazy_background_task_queue() {
-  return nullptr;
-}
-
 EventRouter* MockExtensionSystem::event_router() {
   return event_router_;
 }