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

Source/core/html/StableState.h

Index: Source/core/html/StableState.h
diff --git a/Source/core/html/StableState.h b/Source/core/html/StableState.h
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+// 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.
+
+#ifndef StableState_h
+#define StableState_h
+
+namespace WebCore {
+
+class StableState {
+public:
+    class Awaiter {
+    public:
+        virtual ~Awaiter();
+        // Invoked by provide(). An awaiter may delete itself in the callback,
+        // but must not delete any other awaiter or call await() or provide().
+        virtual void didAwaitStableState() = 0;
+    };
+
+    // Await a stable state. Adding the same awaiter again has no effect. The
+    // only way to stop awaiting a stable state is to delete the awaiter.
+    //
+    // http://whatwg.org/html#await-a-stable-state
+    static void await(Awaiter&);
+
+    // Provide a stable state. Each awaiter will be notified once and then
+    // forgotten.
+    //
+    // http://whatwg.org/html#provide-a-stable-state
+    static void provide();
+
+private:
+    explicit StableState();
+};
+
+}
+
+#endif // StableState_h