Generate subgroup StyleSurroundData in ComputedStyle.

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: {
     // - field_size
     // Number of bits needed to store this field. Only used for storage_only
     // fields. If specified, the field will be stored as bit field. Otherwise
     // it will be stored as a normal data member.
     field_size: {
@@ skipping 146 matching lines @@
     // which can be either a keyword or Length, this is generated as a nonproperty for now. Remove this
     // once we can support union fields and groups.
     {
       name: "VerticalAlign",
       field_template: "storage_only",
       field_size: 4,
       default_value: "EVerticalAlign::kBaseline",
       type_name: "EVerticalAlign",
       has_custom_compare_and_copy: true,
     },
+    {
+      name: "border",
+      field_template: "storage_only",
+      field_type_path: "core/style/BorderData",
+      field_group: "surround",
+      default_value: "BorderData()",
+    },
   ],
 }