Support "await a stable state" and "provide a stable state"

Source/web/WebKit.cpp

Index: Source/web/WebKit.cpp
diff --git a/Source/web/WebKit.cpp b/Source/web/WebKit.cpp
index c2d1c7dfb77d28274624e80b8baf07ff2eea4b01..ee1baa355d4b3f097ea32476d84d9de7a70b3dc0 100644
--- a/Source/web/WebKit.cpp
+++ b/Source/web/WebKit.cpp
@@ -39,8 +39,9 @@
 #include "bindings/v8/V8RecursionScope.h"
 #include "core/Init.h"
 #include "core/dom/Microtask.h"
-#include "core/page/Page.h"
 #include "core/frame/Settings.h"
+#include "core/html/StableState.h"
+#include "core/page/Page.h"
 #include "core/workers/WorkerGlobalScopeProxy.h"
 #include "heap/Heap.h"
 #include "heap/glue/MessageLoopInterruptor.h"
@@ -69,7 +70,9 @@ public:
     virtual void willProcessTask() { }
     virtual void didProcessTask()
     {
+        ASSERT(isMainThread());
         WebCore::Microtask::performCheckpoint();
+        WebCore::StableState::provide();
     }
 };