Revert of Remove the concept of a cleanup task

Source/platform/WebThreadSupportingGC.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "platform/WebThreadSupportingGC.h"
 
 #include "platform/heap/SafePoint.h"
-#include "public/platform/WebScheduler.h"
 #include "wtf/Threading.h"
 
 namespace blink {
 
 PassOwnPtr<WebThreadSupportingGC> WebThreadSupportingGC::create(const char* name)
 {
 #if ENABLE(ASSERT)
     WTF::willCreateThread();
 #endif
     return adoptPtr(new WebThreadSupportingGC(name));
 }
 
 WebThreadSupportingGC::WebThreadSupportingGC(const char* name)
     : m_thread(adoptPtr(Platform::current()->createThread(name)))
 {
 }
 
 WebThreadSupportingGC::~WebThreadSupportingGC()
 {
     if (ThreadState::current()) {
         // WebThread's destructor blocks until all the tasks are processed.
         SafePointScope scope(ThreadState::HeapPointersOnStack);
         m_thread.clear();
     }
 }
 
-void WebThreadSupportingGC::initialize()
+void WebThreadSupportingGC::attachGC()
 {
     m_pendingGCRunner = adoptPtr(new PendingGCRunner);
     m_messageLoopInterruptor = adoptPtr(new MessageLoopInterruptor(&platformThread()));
     platformThread().addTaskObserver(m_pendingGCRunner.get());
     ThreadState::attach();
     ThreadState::current()->addInterruptor(m_messageLoopInterruptor.get());
 }
 
-void WebThreadSupportingGC::shutdown()
+void WebThreadSupportingGC::detachGC()
 {
     ThreadState::current()->removeInterruptor(m_messageLoopInterruptor.get());
     ThreadState::detach();
     platformThread().removeTaskObserver(m_pendingGCRunner.get());
     m_pendingGCRunner = nullptr;
     m_messageLoopInterruptor = nullptr;
-
-    // Ensure no posted tasks will run from this point on.
-    platformThread().scheduler()->shutdown();
 }
 
 } // namespace blink