Generate StyleBackgroundData in ComputedStyleBase.

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 151 matching lines @@
       default_value: "EVerticalAlign::kBaseline",
       type_name: "EVerticalAlign",
     },
     {
       name: "border",
       field_template: "storage_only",
       field_type_path: "core/style/BorderData",
       field_group: "surround",
       default_value: "BorderData()",
     },
+    {
+      name: "Background",
+      field_template: "storage_only",
+      field_type_path: "core/style/FillLayer",
+      default_value: "FillLayer(kBackgroundFillLayer, true)",
+      field_group: "background",
+    },
+    {
+      name: "BackgroundColor",
+      field_template: "storage_only",
+      field_type_path: "core/css/StyleColor",
+      default_value: "Color::kTransparent",
+      field_group: "background",
+    },
   ],
 }