Add keyword_subset field template and generate text-decoration-line.

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 69 matching lines @@
     // - field_template
     // Affects how the interface to this field is generated.
     // TODO(sashab, meade): Remove this once TypedOM types are specified for
     // every property, since this value can be inferred from that.
     field_template: {
       valid_values: [
         // Field is stored as an enum and has a initial/getter/setter/resetter.
         // If include_paths is empty, we would also generate the corresponding
         // enum definition in ComputedStyleConstants.h.
         "keyword",
+        // Field can take on any subset of values from a list of keywords.
+        "multi_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 type_name 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.
@@ skipping 2446 matching lines @@
       field_group: "rare-non-inherited",
       default_value: "StyleColor::CurrentColor()",
     },
     {
       name: "text-decoration-line",
       api_class: "CSSPropertyAPITextDecorationLine",
       converter: "ConvertFlags<TextDecoration>",
       name_for_methods: "TextDecoration",
       runtime_flag: "CSS3TextDecorations",
       type_name: "TextDecoration",
-      field_template: "storage_only",
-      field_size: 4,
-      default_value: "TextDecoration::kNone",
+      field_template: "multi_keyword",
+      default_value: "none",
+      keywords: ["none", "underline", "overline", "line-through", "blink"],
       field_group: "visual",
     },
     {
       name: "text-decoration-skip",
       api_class: true,
       api_methods: ["parseSingleValue"],
       converter: "ConvertFlags<TextDecorationSkip>",
       inherited: true,
       runtime_flag: "CSS3TextDecorations",
       field_template: "storage_only",
@@ skipping 1645 matching lines @@
     {
       name: "-webkit-transition-timing-function",
       alias_for: "transition-timing-function",
     },
     {
       name: "-webkit-user-select",
       alias_for: "user-select",
     },
   ],
 }