Switch SupportsUserData uses to use unique_ptr.

ios/web/web_state/ui/wk_back_forward_list_item_holder.h

Index: ios/web/web_state/ui/wk_back_forward_list_item_holder.h
diff --git a/ios/web/web_state/ui/wk_back_forward_list_item_holder.h b/ios/web/web_state/ui/wk_back_forward_list_item_holder.h
index 793f311c890e52bed293fdc8fd7878a52f1433f1..f2b041d6e17a9f59232359da55a47081e13bdfb5 100644
--- a/ios/web/web_state/ui/wk_back_forward_list_item_holder.h
+++ b/ios/web/web_state/ui/wk_back_forward_list_item_holder.h
@@ -23,6 +23,8 @@ class NavigationItem;
 // thus needs to be preserved (e.g., WKNavigationType, MIME type).
 class WKBackForwardListItemHolder : public base::SupportsUserData::Data {
  public:
+  ~WKBackForwardListItemHolder() override;
+
   // Returns the WKBackForwardListItemHolder for the NavigationItem |item|.
   // Lazily attaches one if it does not exist. |item| cannot be null.
   static web::WKBackForwardListItemHolder* FromNavigationItem(
@@ -56,7 +58,6 @@ class WKBackForwardListItemHolder : public base::SupportsUserData::Data {
 
  private:
   WKBackForwardListItemHolder();
-  ~WKBackForwardListItemHolder() override;
 
   // Weak pointer to a WKBackForwardListItem. Becomes nil if the parent
   // WKBackForwardList is deallocated.