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 0bd01ff2cda7ac1e519c092696360d568280ffa0..8371c951401649b1efe51fbbbce8ae50049a4fb4 100644
--- a/extensions/browser/mock_extension_system.cc
+++ b/extensions/browser/mock_extension_system.cc
@@ -46,10 +46,6 @@ InfoMap* MockExtensionSystem::info_map() {
   return nullptr;
 }
 
-LazyBackgroundTaskQueue* MockExtensionSystem::lazy_background_task_queue() {
-  return nullptr;
-}
-
 EventRouter* MockExtensionSystem::event_router() {
   return event_router_;
 }