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 e5dd4680033c715d0feeb1f01c861fe38908d2bf..71080a63bf4c7ecd75f674ea4c483de8cecb1bbc 100644
--- a/extensions/browser/extension_system.h
+++ b/extensions/browser/extension_system.h
@@ -36,7 +36,6 @@ class Extension;
 class ExtensionSet;
 class InfoMap;
 class InstallVerifier;
-class LazyBackgroundTaskQueue;
 class ManagementPolicy;
 class OneShotEvent;
 class QuotaService;
@@ -88,9 +87,6 @@ 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;