Makes descriptors hold nullptr for properties not implemented in API.

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"
 
 namespace blink {
 
 class CSSValue;
 class CSSParserContext;
 class CSSParserTokenRange;
 
 // We will use this API to represent all functions used for property-specific
 // logic inside the blink style engine. All specific properties are subclasses
 // of CSSPropertyAPI.
 //
 // To add a new implementation of this API for a property:
 // - Make a class that implements CSSPropertyAPI.
 // - For each method that you wish to implement in this class, add this method
 //   name to the api_methods flag in CSSProperties.json5.
 // - Implement these methods in the .cpp file.
 //
 // To add new functions to this API:
 // - Add the function to the struct below.
-// - Add the function to CSSPropertyAPIFiles.h.tmpl template file, surrounded
-//   with an {% if "methodName" in implementedMethods %} check.
-// - Add the function name to the valid_methods array in
-//   make_css_property_apis.py.
-// - Add function to the initializer list in the cssPropertyDescriptors array
-//   in CSSPropertyDescriptor.cpp.tmpl, and add a nullptr to the invalid
-//   CSSPropertyDescriptor.
+// - Add the function name to the valid_values field for api_methods in
+//   CSSProperties.json5.

[sashab, 2017/02/01 17:50:20]

Can you add a comment to CSSProperties.json5 too, since we now have a weird use
for valid values, like:

"api_methods": {
  // This list must be an ordered, complete list of methods whose names match
those defined in CSSPropertyDescriptor.h.
  "valid_values"

[aazzam, 2017/02/01 22:33:16]

done :)

 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*) {
     // No code should reach here, since properties either have their own
     // implementations of this method or store nullptr in their descriptor.
     NOTREACHED();
     return nullptr;
   }
 };
 
 }  // namespace blink
 
 #endif  // CSSPropertyAPI_h