Clean up forward declarations in Source/core

Source/core/dom/StyleChangeReason.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 StyleChangeReason_h
 #define StyleChangeReason_h
 
 #include "core/dom/QualifiedName.h"
 #include "wtf/NullPtr.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/text/AtomicString.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 class QualifiedName;
-namespace TraceEvent {
-class ConvertableToTraceFormat;
-}
 
 namespace StyleChangeReason {
 extern const char ActiveStylesheetsUpdate[];
 extern const char Animation[];
 extern const char Attribute[];
 extern const char ControlValue[];
 extern const char Control[];
 extern const char DesignMode[];
 extern const char Drag[];
 extern const char FontSizeChange[];
@@ skipping 68 matching lines @@
     void operator==(const StyleChangeReasonForTracing&) const { }
     void operator!=(const StyleChangeReasonForTracing&) const { }
 
     const char* m_reason;
     AtomicString m_extraData;
 };
 
 } // namespace blink
 
 #endif // StyleChangeReason_h