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
new file mode 100644
index 0000000000000000000000000000000000000000..764cdd5c3ab6d9ec0acf892b0a1cf9ad17caa884
--- /dev/null
+++ b/Source/core/html/StableState.h
@@ -0,0 +1,36 @@
+// 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();
+        virtual void didAwaitStableState() = 0;

[acolwell GONE FROM CHROMIUM, 2014/02/10 22:36:05]

nit: didAwaitStableState seems like an awkward name. How about something like
onStableState, reachedStableState, awaitDone, or awaitComplete?

[philipj_slow, 2014/02/11 17:25:21]

I rather like my name, actually, it blends well with other things in
HTMLMediaElement like didMoveToNewDocument, didRecalcStyle, didAddTrack,
didRemoveTrack and didBecomeFullscreenElement.

Of course, if you pick a specific name you would prefer, I will make it so.

[acolwell GONE FROM CHROMIUM, 2014/02/18 18:47:42]

You're right. I retract my objection. I'm fine with keeping this as is.

+    };
+
+    // 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. Calling await() inside the callback is not allowed.
+    //
+    // http://whatwg.org/html#provide-a-stable-state
+    static void provide();
+
+private:
+    explicit StableState();
+};
+
+}
+
+#endif // StableState_h