Generate getters/setters for border-image.

Currently, the border-image property is a 'storage_only' field, so
it has no generated public getters/setters. This patch changes it to an
'external' field so that their getters/setters can be generated as well.

Diff: https://gist.github.com/dc683ef590e3d4e88e1ed7f3a54ff16f/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2888323002
Cr-Commit-Position: refs/heads/master@{#473403}

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

@@ -2185,7 +2185,6 @@
       name: "-webkit-border-image",
       api_class: true,
       custom_value: true,
-      initial: "InitialNinePieceImage",
     },
     {
       name: "-webkit-border-vertical-spacing",

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

@@ -191,7 +191,7 @@
     },
     {
       name: "border-image",
-      field_template: "storage_only",
+      field_template: "external",
       field_type_path: "core/style/NinePieceImage",
       field_group: "surround",
       default_value: "NinePieceImage()",

third_party/WebKit/Source/core/style/ComputedStyle.h

@@ -426,11 +426,6 @@ class CORE_EXPORT ComputedStyle : public ComputedStyleBase<ComputedStyle>,
   }
 
   // Border properties.
-  // -webkit-border-image
-  static NinePieceImage InitialNinePieceImage() { return NinePieceImage(); }
-  const NinePieceImage& BorderImage() const { return BorderImageInternal(); }
-  void SetBorderImage(const NinePieceImage& b) { SetBorderImageInternal(b); }
-
   // border-image-slice
   const LengthBox& BorderImageSlices() const {
     return BorderImage().ImageSlices();
@@ -2972,7 +2967,6 @@ class CORE_EXPORT ComputedStyle : public ComputedStyleBase<ComputedStyle>,
     SetBorderLeftColorInternal(0);
     SetBorderLeftColorInternal(true);
   }
-  void ResetBorderImage() { SetBorderImageInternal(NinePieceImage()); }
 
   void SetBorderRadius(const LengthSize& s) {
     SetBorderTopLeftRadius(s);