Extract LazyBackgroundTaskQueue from ExtensionSystem.

extensions/browser/extension_system.h

Index: extensions/browser/extension_system.h
diff --git a/extensions/browser/extension_system.h b/extensions/browser/extension_system.h
index ec4c71687d1aede01a400d32099bccdea0ef5937..600ac3a8fbc7607d96b845f31b48592d91429149 100644
--- a/extensions/browser/extension_system.h
+++ b/extensions/browser/extension_system.h
@@ -34,7 +34,6 @@ class EventRouter;
 class Extension;
 class ExtensionSet;
 class InfoMap;
-class LazyBackgroundTaskQueue;
 class ManagementPolicy;
 class OneShotEvent;
 class QuotaService;
@@ -83,9 +82,6 @@ class ExtensionSystem : public KeyedService {
   // Returns the IO-thread-accessible extension data.
   virtual InfoMap* info_map() = 0;
 
-  // The LazyBackgroundTaskQueue is created at startup.
-  virtual LazyBackgroundTaskQueue* lazy_background_task_queue() = 0;
-
   // The EventRouter is created at startup.
   virtual EventRouter* event_router() = 0;