[Typed OM] Add CSSValue -> CSSNumberValue conversion

third_party/WebKit/Source/core/css/cssom/CSSNumberValue.h

 // Copyright 2015 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 CSSNumberValue_h
 #define CSSNumberValue_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/CoreExport.h"
 #include "core/css/CSSPrimitiveValue.h"
@@ skipping 19 matching lines @@
   }
 
   StyleValueType type() const override { return StyleValueType::NumberType; }
 
  private:
   CSSNumberValue(double value) : m_value(value) {}
 
   double m_value;
 };
 
+DEFINE_TYPE_CASTS(CSSNumberValue,
+                  CSSStyleValue,
+                  value,
+                  value->type() == CSSStyleValue::NumberType,
+                  value.type() == CSSStyleValue::NumberType);
+
 }  // namespace blink
 
 #endif