Allow ComputedStyleBase property fields to be storage_only.

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 60 matching lines @@
         "keyword",
         // Field stores a primitive value like int/bool. The type is specified by
         // type_name. The interface has a initial/getter/setter/resetter.
         "primitive",
         // Field is stored as a bool, whose default value is false
         // and can only be set to true. Has a initial/getter/setter.
         "monotonic_flag",
         // Field has type specified at field_type_path and has a getter/setter.
         // Also has a setter taking an rvalue reference. Cannot be packed.
         "external",
+        // Only the storage is generated. The interface is handwritten.
+        "storage_only",
       ],
     },
 
     // - field_type_path: "path/to/Type"
     // For properties that have generated field storage in ComputedStyle,
     // this optional argument will override the field's generated type with
     // an external one specified at the given path. The type must be defined
     // in a header file at that path, and have the same name as the file.
     // Currently, only enum types are supported, and the enum's only values
     // must be CamelCase values of the keywords of the property.
@@ skipping 3102 matching lines @@
     {
       name: "-webkit-transition-timing-function",
       alias_for: "transition-timing-function",
     },
     {
       name: "-webkit-user-select",
       alias_for: "user-select",
     },
   ],
 }