Revert of Change the update_client task runner behavior to continue on shutdown.

components/update_client/task.h

Index: components/update_client/task.h
diff --git a/components/update_client/task.h b/components/update_client/task.h
index 0a5b7f32f92f77397917f14af56aec0737b80941..a6d1f6c61948805ec0cc1e00cd852ea812bef658 100644
--- a/components/update_client/task.h
+++ b/components/update_client/task.h
@@ -26,11 +26,6 @@
 
   virtual void Run() = 0;
 
-  // Does a best effort attempt to make a task release its resources and stop
-  // soon. It is possible that a running task may complete even if this
-  // method is called.
-  virtual void Cancel() = 0;
-
   // Returns the ids corresponding to the CRXs associated with this update task.
   virtual std::vector<std::string> GetIds() const = 0;
 };