Switch SupportsUserData uses to use unique_ptr.

ios/web/web_state/ui/wk_back_forward_list_item_holder.mm

Index: ios/web/web_state/ui/wk_back_forward_list_item_holder.mm
diff --git a/ios/web/web_state/ui/wk_back_forward_list_item_holder.mm b/ios/web/web_state/ui/wk_back_forward_list_item_holder.mm
index 0e99baab88f97701e26ee87f8d4c208a8849218e..f74be49f6fbc9e237c80dd3762e835ca14af25d8 100644
--- a/ios/web/web_state/ui/wk_back_forward_list_item_holder.mm
+++ b/ios/web/web_state/ui/wk_back_forward_list_item_holder.mm
@@ -4,6 +4,7 @@
 
 #import "ios/web/web_state/ui/wk_back_forward_list_item_holder.h"
 
+#include "base/memory/ptr_util.h"
 #import "ios/web/public/navigation_item.h"
 
 namespace web {
@@ -29,7 +30,8 @@ WKBackForwardListItemHolder* WKBackForwardListItemHolder::FromNavigationItem(
       item->GetUserData(kBackForwardListItemIdentifierKey);
   if (!user_data) {
     user_data = new WKBackForwardListItemHolder();
-    item->SetUserData(kBackForwardListItemIdentifierKey, user_data);
+    item->SetUserData(kBackForwardListItemIdentifierKey,
+                      base::WrapUnique(user_data));
   }
   return static_cast<WKBackForwardListItemHolder*>(user_data);
 }