Remove unnecessary Document.h includes in .cpp files

Source/core/dom/MutationObserver.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 16 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/dom/MutationObserver.h"
 
 #include <algorithm>
 #include "bindings/v8/Dictionary.h"
 #include "bindings/v8/ExceptionState.h"
-#include "core/dom/Document.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/Microtask.h"
 #include "core/dom/MutationCallback.h"
 #include "core/dom/MutationObserverRegistration.h"
 #include "core/dom/MutationRecord.h"
 #include "core/dom/Node.h"
 #include "core/inspector/InspectorInstrumentation.h"
 #include "wtf/MainThread.h"
 
 namespace WebCore {
@@ skipping 237 matching lines @@
     }
 }
 
 void MutationObserver::trace(Visitor* visitor)
 {
     visitor->trace(m_records);
     visitor->trace(m_registrations);
 }
 
 } // namespace WebCore