Generate enum/getters/setters/mappings for box-sizing.

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 the field has_custom_compare_and_copy, then it does not appear in
     // ComputedStyle::operator== and ComputedStyle::CopyNonInheritedFromCached.
     has_custom_compare_and_copy: {
       default: false,
     },
@@ skipping 224 matching lines @@
       default_value: "true",
     },
     {
       name: "BoxDecorationBreak",
       field_template: "storage_only",
       type_name: "EBoxDecorationBreak",
       field_size: 1,
       field_group: "box",
       default_value: "EBoxDecorationBreak::kSlice",
     },
-    {
-      name: "BoxSizing",
-      field_template: "storage_only",
-      type_name: "EBoxSizing",
-      field_size: 1,
-      field_group: "box",
-      default_value: "EBoxSizing::kContentBox",
-    },
   ],
 }