Refactor TimeBase to post tasks. Workers to use real Idle tasks.

Source/core/workers/WorkerGlobalScope.cpp

Index: Source/core/workers/WorkerGlobalScope.cpp
diff --git a/Source/core/workers/WorkerGlobalScope.cpp b/Source/core/workers/WorkerGlobalScope.cpp
index ad0e8eacdad9a1300021b007c10bb0cd079a4d52..db5904a4dc4002ba9b603c929399e936c5f7bb62 100644
--- a/Source/core/workers/WorkerGlobalScope.cpp
+++ b/Source/core/workers/WorkerGlobalScope.cpp
@@ -183,6 +183,7 @@ void WorkerGlobalScope::close()
     // After m_closing is set, all the tasks in the queue continue to be fetched but only
     // tasks with isCleanupTask()==true will be executed.
     m_closing = true;
+    m_thread->preShutdown();
     postTask(FROM_HERE, CloseWorkerGlobalScopeTask::create());
 }