Move NoEventDispatchAssertion class to its own file

Source/core/dom/NoEventDispatchAssertion.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 NoEventDispatchAssertion_h
+#define NoEventDispatchAssertion_h
+
+#include "wtf/MainThread.h"
+#include "wtf/TemporaryChange.h"
+
+namespace WebCore {
+
+#ifndef NDEBUG
+
+class NoEventDispatchAssertion {
+public:
+    NoEventDispatchAssertion()
+    {
+        if (!isMainThread())
+            return;
+        ++s_count;
+    }
+
+    ~NoEventDispatchAssertion()
+    {
+        if (!isMainThread())
+            return;
+        ASSERT(s_count);
+        --s_count;
+    }
+
+    static bool isEventDispatchForbidden()
+    {
+        if (!isMainThread())
+            return false;
+        return s_count;
+    }
+
+    // It's safe to dispatch events in SVGImage since there can't be any script
+    // listeners.
+    class AllowSVGImageEvents {
+    public:
+        AllowSVGImageEvents()
+            : m_change(s_count, 0)
+        {
+        }
+
+        ~AllowSVGImageEvents()
+        {
+            ASSERT(!s_count);
+        }
+
+        TemporaryChange<unsigned> m_change;
+    };
+
+private:
+    static unsigned s_count;
+};
+
+#else
+
+class NoEventDispatchAssertion {
+public:
+    NoEventDispatchAssertion() { }
+
+    class AllowSVGImageEvents {
+    public:
+        AllowSVGImageEvents() { }
+    };
+};
+
+#endif // NDEBUG
+
+} // namespace WebCore
+
+#endif // NoEventDispatchAssertion_h