Have srcset respond to viewport changes

Source/core/frame/ViewportChangeNotifier.cpp

+// 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/frame/ViewportChangeNotifier.h"
+
+namespace WebCore {
+
+DEFINE_EMPTY_DESTRUCTOR_WILL_BE_REMOVED(ViewportChangeListener);
+
+PassOwnPtrWillBeRawPtr<ViewportChangeNotifier> ViewportChangeNotifier::create()
+{
+    return adoptPtrWillBeNoop(new ViewportChangeNotifier());
+}
+
+void ViewportChangeNotifier::addListener(ViewportChangeListener* listener)
+{
+    m_listeners.add(listener);
+}
+
+void ViewportChangeNotifier::removeListener(ViewportChangeListener* listener)
+{
+    ListenerContainer::iterator it = m_listeners.find(listener);
+    if (it != m_listeners.end())
+        m_listeners.remove(it);
+}
+
+void ViewportChangeNotifier::viewportChanged()
+{
+    for (ListenerContainer::iterator it = m_listeners.begin(); it != m_listeners.end(); ++it)
+        (*it)->viewportChanged();
+}
+
+void ViewportChangeNotifier::trace(Visitor* visitor)
+{
+    visitor->trace(m_listeners);
+}
+
+} // namespace