Imported Upstream version 4.8.0.309

Former-commit-id: 5f9c6ae75f295e057a7d2971f3a6df4656fa8850

mcs/class/referencesource/System.Web/Util/SynchronizationHelper.cs

@@ -105,7 +105,7 @@ namespace System.Web.Util {
             // This method only schedules work; it doesn't itself do any work. The lock is held for a very
             // short period of time.
             lock (_lockObj) {
-                Task newTask = _lastScheduledTask.ContinueWith(_ => SafeWrapCallback(action));
+                Task newTask = _lastScheduledTask.ContinueWith(_ => SafeWrapCallback(action), TaskScheduler.Default);
                 _lastScheduledTask = newTask; // the newly-created task is now the last one
             }
         }