Remove the concept of a cleanup task

Source/platform/WebThreadSupportingGC.cpp

Index: Source/platform/WebThreadSupportingGC.cpp
diff --git a/Source/platform/WebThreadSupportingGC.cpp b/Source/platform/WebThreadSupportingGC.cpp
index cac7795876ae25a36bad6e981ec72474b866e6eb..256552193e30ec08803eed7793b5c94dedfee127 100644
--- a/Source/platform/WebThreadSupportingGC.cpp
+++ b/Source/platform/WebThreadSupportingGC.cpp
@@ -6,6 +6,7 @@
 #include "platform/WebThreadSupportingGC.h"
 
 #include "platform/heap/SafePoint.h"
+#include "public/platform/WebScheduler.h"
 #include "wtf/Threading.h"
 
 namespace blink {
@@ -32,7 +33,7 @@ WebThreadSupportingGC::~WebThreadSupportingGC()
     }
 }
 
-void WebThreadSupportingGC::attachGC()
+void WebThreadSupportingGC::initialize()
 {
     m_pendingGCRunner = adoptPtr(new PendingGCRunner);
     m_messageLoopInterruptor = adoptPtr(new MessageLoopInterruptor(&platformThread()));
@@ -41,13 +42,19 @@ void WebThreadSupportingGC::attachGC()
     ThreadState::current()->addInterruptor(m_messageLoopInterruptor.get());
 }
 
-void WebThreadSupportingGC::detachGC()
+void WebThreadSupportingGC::shutdown()
 {
+    // Ensure no posted tasks will run from this point on.
+    platformThread().removeTaskObserver(m_pendingGCRunner.get());
+    platformThread().scheduler()->shutdown();
+
     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