Added api_methods flag to CSSProperties.json5.

third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h

 // 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 CSSPropertyAPI_h
 #define CSSPropertyAPI_h
 
 #include "core/CSSPropertyNames.h"
 #include "wtf/Allocator.h"
 
@@ skipping 19 matching lines @@
 // - When adding new functions, also add them to GET_DESCRIPTOR, and the get()
 //   method in CSSPropertyDescriptors.cpp, and the descriptor struct in
 //   CSSPropertyDescriptor.h.
 class CSSPropertyAPI {
   STATIC_ONLY(CSSPropertyAPI);
 
  public:
   // Parses a single CSS property and returns the corresponding CSSValue. If the
   // input is invalid it returns nullptr.
   static const CSSValue* parseSingleValue(CSSParserTokenRange&,
-                                          const CSSParserContext*);
+                                          const CSSParserContext*) {
+    NOTREACHED();
+    return nullptr;
+  }
 };
 
 }  // namespace blink
 
 #endif  // CSSPropertyAPI_h