Revert of Remove the concept of a cleanup task

public/platform/WebScheduler.h

Index: public/platform/WebScheduler.h
diff --git a/public/platform/WebScheduler.h b/public/platform/WebScheduler.h
index 2e3760e6902a52b3b73ab3869448a639e9f6c8c5..463d1300af735ada6a105b97c63716c5c66145e6 100644
--- a/public/platform/WebScheduler.h
+++ b/public/platform/WebScheduler.h
@@ -17,10 +17,6 @@
 class BLINK_PLATFORM_EXPORT WebScheduler {
 public:
     virtual ~WebScheduler() { }
-
-    // Called to prevent any more pending tasks from running. Must be called on
-    // the associated WebThread.
-    virtual void shutdown() { }
 
     // Returns true if there is high priority work pending on the associated WebThread
     // and the caller should yield to let the scheduler service that work.