Fix crash in webmidi.

Source/modules/webmidi/MIDIPortMap.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 MIDIPortMap_h
 #define MIDIPortMap_h
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "bindings/core/v8/ScriptValue.h"
@@ skipping 13 matching lines @@
 
     // IDL attributes / methods
     size_t size() const { return m_entries.size(); }
     Iterator* keys();
     Iterator* entries();
     Iterator* values();
     T* get(const String& key) const;
     bool has(const String& key) const { return m_entries.contains(key); }
     Iterator* iterator(ScriptState*, ExceptionState&) { return entries(); }
 
-    void trace(Visitor* visitor)
+    virtual void trace(Visitor* visitor)
     {
         visitor->trace(m_entries);
     }
 
 private:
     typedef HeapHashMap<String, Member<T> > MapType;
     typedef typename HeapHashMap<String, Member<T> >::const_iterator IteratorType;
     struct KeySelector {
         static const String& select(ScriptState*, IteratorType i) { return i->key; }
     };
@@ skipping 73 matching lines @@
 
 template <typename T>
 T* MIDIPortMap<T>::get(const String& key) const
 {
     return has(key) ? m_entries.get(key) : 0;
 }
 
 } // namespace blink
 
 #endif