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

Source/core/html/StableState.h

+// 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 {
+
+// http://whatwg.org/html#await-a-stable-state
+// http://whatwg.org/html#provide-a-stable-state
+class StableState {
+public:
+    class Awaiter {
+    public:
+        virtual ~Awaiter();
+        virtual void didAwaitStableState() = 0;
+
+        void awaitStableState();

[philipj_slow, 2014/02/04 13:45:46]

I put this here to make the call sites slightly prettier, but it could just as
well be static StableState::await(Awaiter&). That would keep the static bits
together, instead of hiding the fact as I've done now. Should I just change it?

[philipj_slow, 2014/02/04 15:10:49]

I've made it so in PS2, so you can see exactly what the options are.

+        void stopAwaitingStableState();
+    };
+
+    static void provide();
+
+private:
+    explicit StableState();
+};
+
+}
+
+#endif // StableState_h