Makes descriptors hold nullptr for properties not implemented in API.

third_party/WebKit/Source/core/css/CSSProperties.json5

 {
 // This file specifies all the CSS properties we support and the necessary
 // information for our code generation. The various supported arguments
 // are described below with example usage
 
   parameters: {
     // - alias_for: "other-property"
     // Properties specifying alias_for should be virtually identical to the
     // properties they alias. Minor parsing differences are allowed as long as
     // the CSSValues created are of the same format of the aliased property.
@@ skipping 32 matching lines @@
     },
 
     // - api_methods: ["method1", "method2"]
     // List of methods that are implemented in the CSSPropertyAPI for this property.
     // This is used by make_css_property_apis.py to specify which functions are defined
     // in the .h file for this property, and also used in CSSPropertyAPIFiles.h.tmpl to
     // generate declarations for only the methods this property has implementations for.
     api_methods: {
       default: [],
       valid_type: "list",
+       // This list must be an ordered, complete list of methods whose names match those
+       // defined in CSSPropertyDescriptor.h.
       valid_values: ["parseSingleValue"],
     },
 
     // - keyword_only
     // These properties only store keyword values. This is used when
     // generating the ComputedStyle storage for the property. The initial
     // value for this property on a ComputedStyle is specified with the
     // initial_keyword flag below.
     // TODO(sashab): Rename this to field_type=keyword once we support
     // multiple types of generatable fields in ComputedStyle
@@ skipping 2869 matching lines @@
     {
       name: "-webkit-transition-timing-function",
       alias_for: "transition-timing-function",
     },
     {
       name: "-webkit-user-select",
       alias_for: "user-select",
     },
   ],
 }