Added api_methods flag to CSSProperties.json5.

third_party/WebKit/Source/build/scripts/templates/CSSPropertyAPIFiles.h.tmpl

 // Copyright 2016 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 {{api_classname}}_h
 #define {{api_classname}}_h
 
 #include "core/CSSPropertyNames.h"
 #include "core/css/properties/CSSPropertyAPI.h"
 
 namespace blink {
 
 class CSSParserTokenRange;
 class CSSParserContext;
 
 class {{api_classname}} : public CSSPropertyAPI {
  public:
+  {% if "parseSingleValue" in api_methods %}
   static const CSSValue* parseSingleValue(CSSParserTokenRange&,
                                           const CSSParserContext*);
+  {% endif %}
 };
 
 }  // namespace blink
 
 #endif  // {{api_classname}}_h