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
new file mode 100644
index 0000000000000000000000000000000000000000..f6e7d85fd7b8df918a49e39fbcf0262d2df3f1c6
--- /dev/null
+++ b/Source/core/html/StableState.cpp
@@ -0,0 +1,43 @@
+// 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"
+
+namespace WebCore {
+
+typedef HashSet<StableState::Awaiter*> StableStateAwaiters;
+
+static StableStateAwaiters& stableStateAwaiters()
+{
+    DEFINE_STATIC_LOCAL(StableStateAwaiters, stableStateAwaiters, ());
+    return stableStateAwaiters;
+}
+
+StableState::Awaiter::~Awaiter()
+{
+    stopAwaitingStableState();
+}
+
+void StableState::Awaiter::awaitStableState()
+{
+    stableStateAwaiters().add(this);
+}
+
+void StableState::Awaiter::stopAwaitingStableState()
+{
+    stableStateAwaiters().remove(this);
+}
+
+void StableState::provide()
+{
+    StableStateAwaiters& awaiters = stableStateAwaiters();
+    for (StableStateAwaiters::iterator it = awaiters.begin(); it != awaiters.end(); ++it)
+        (*it)->didAwaitStableState();
+    awaiters.clear();
+}
+
+} // namespace WebCore