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

Source/core/html/StableState.cpp

Index: Source/core/html/StableState.cpp
diff --git a/Source/core/html/StableState.cpp b/Source/core/html/StableState.cpp
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index 0000000000000000000000000000000000000000..761917c2e5956e1b71febbbaedde925056d1ac22
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+++ b/Source/core/html/StableState.cpp
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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.
+
+#include "config.h"
+#include "core/html/StableState.h"
+
+#include "wtf/HashSet.h"
+#include "wtf/MainThread.h"
+
+namespace WebCore {
+
+class StableStateAwaiters {
+public:
+    StableStateAwaiters() : m_currentAwaiter(0) { }
+
+    void add(StableState::Awaiter* awaiter)
+    {
+        ASSERT(!m_currentAwaiter);
+        m_awaiters.add(awaiter);
+    }
+
+    void remove(StableState::Awaiter* awaiter)
+    {
+        ASSERT(!m_currentAwaiter || m_currentAwaiter == awaiter);
+        m_awaiters.remove(awaiter);
+    }
+
+    void didAwaitStableState()
+    {
+        ASSERT(!m_currentAwaiter);
+        while (!m_awaiters.isEmpty()) {
+            m_currentAwaiter = *m_awaiters.begin();
+            m_currentAwaiter->didAwaitStableState();
+            remove(m_currentAwaiter);
+            m_currentAwaiter = 0;
+        }
+    }
+
+private:
+    HashSet<StableState::Awaiter*> m_awaiters;
+    StableState::Awaiter* m_currentAwaiter;
+};
+
+static StableStateAwaiters& stableStateAwaiters()
+{
+    ASSERT(isMainThread());
+    DEFINE_STATIC_LOCAL(StableStateAwaiters, stableStateAwaiters, ());
+    return stableStateAwaiters;
+}
+
+StableState::Awaiter::~Awaiter()
+{
+    stableStateAwaiters().remove(this);
+}
+
+void StableState::await(Awaiter& awaiter)
+{
+    stableStateAwaiters().add(&awaiter);
+}
+
+void StableState::provide()
+{
+    stableStateAwaiters().didAwaitStableState();
+}
+
+} // namespace WebCore