Update StyleValue.cssString() to use existing methods instead of rolling our own.

third_party/WebKit/Source/core/css/cssom/NumberValue.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 NumberValue_h
 #define NumberValue_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/CoreExport.h"
 #include "core/css/CSSPrimitiveValue.h"
@@ skipping 10 matching lines @@
     {
         return new NumberValue(value);
     }
 
     double value() const { return m_value; }
     void setValue(double value)
     {
         m_value = value;
     }
 
-    String cssString() const override { return String::number(m_value); }
-
     PassRefPtrWillBeRawPtr<CSSValue> toCSSValue() const override
     {
         return cssValuePool().createValue(m_value, CSSPrimitiveValue::UnitType::
 Number);
     }
 
     StyleValueType type() const override { return StyleValueType::NumberType; }
 private:
     NumberValue(double value) : m_value(value) {}
 
     double m_value;
 };
 
 } // namespace blink
 
 #endif