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

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

 {
 // This file specifies fields in ComputedStyle that we would like to
 // generate, but are not CSS properties.
 
   parameters: {
     // If true, we do not include this field in ComputedStyleBase::InheritFrom
     // and ComputedStyleBase::CopyNonInheritedFromCached.
     custom_copy: {
       default: false,
     },
@@ skipping 213 matching lines @@
     },
     {
       name: "BackgroundColor",
       field_template: "storage_only",
       include_paths: ["core/css/StyleColor.h"],
       type_name: "StyleColor",
       default_value: "Color::kTransparent",
       field_group: "background",
     },
     {
-      name: "TextDecoration",
-      field_template: "storage_only",
-      type_name: "TextDecoration",
-      field_size: 4,
-      default_value: "TextDecoration::kNone",
-      field_group: "visual",
-    },
-    {
       name: "HasAutoClip",
       field_template: "storage_only",
       type_name: "bool",
       field_size: 1,
       default_value: "true",
       field_group: "visual",
     },
     {
       name: "HasAutoZIndex",
       field_template: "storage_only",
@@ skipping 314 matching lines @@
       inherited: true,
       field_template: "storage_only",
       type_name: "StyleInheritedVariables",
       include_paths: ["core/style/StyleInheritedVariables.h"],
       default_value: "nullptr",
       wrapper_pointer_name: "RefPtr",
       field_group: "rare-inherited",
     },
   ],
 }