Moved overflow-x and overflow-y to be generated in ComputedStyleBase.

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 1337 matching lines @@
       keywords: [
         "visible", "none", "auto",
       ],
     },
     {
       name: "overflow-wrap",
       inherited: true,
     },
     {
       name: "overflow-x",
+      initial_keyword: "visible",
+      keyword_only: true,
+      keywords: [
+        "visible", "hidden", "scroll", "auto", "overlay", "-webkit-paged-x", "-webkit-paged-y",
+      ],

[sashab, 2017/02/03 05:05:23]

type_name: EOverflow

       type_name: "EOverflow",
     },
     {
       name: "overflow-y",
+      initial_keyword: "visible",
+      keyword_only: true,
+      keywords: [
+        "visible", "hidden", "scroll", "auto", "overlay", "-webkit-paged-x", "-webkit-paged-y",
+      ],

[sashab, 2017/02/03 05:05:23]

type_name: EOverflow

       type_name: "EOverflow",
     },
     {
       name: "padding-bottom",
       api_class: "CSSPropertyAPIPadding",
       api_methods: ["parseSingleValue"],
       converter: "convertLength",
       initial: "initialPadding",
       interpolable: true,
     },
@@ skipping 1590 matching lines @@
     {
       name: "-webkit-transition-timing-function",
       alias_for: "transition-timing-function",
     },
     {
       name: "-webkit-user-select",
       alias_for: "user-select",
     },
   ],
 }