Makes descriptors hold nullptr for properties not implemented in API.

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

Index: third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h
diff --git a/third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h b/third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h
index 938a6e0e977603950a0aadbb1591851e1e270c07..66b49c1d847ce8648f210b6f4cb9f3478f68e1eb 100644
--- a/third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h
+++ b/third_party/WebKit/Source/core/css/properties/CSSPropertyAPI.h
@@ -26,13 +26,8 @@ class CSSParserTokenRange;
 //
 // 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);