LibWeb: Use shorter variant of `fill_rect_with_rounded_corners()`

Instead of passing `thumb_corner_radius` 4 times, pass it only once.

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -343,11 +343,11 @@ void PaintableBox::paint(PaintContext& context, PaintPhase phase) const
         int thumb_corner_radius = static_cast<int>(context.rounded_device_pixels(scrollbar_thumb_thickness / 2));
         if (auto thumb_rect = scroll_thumb_rect(ScrollDirection::Horizontal); thumb_rect.has_value()) {
             auto thumb_device_rect = context.enclosing_device_rect(thumb_rect.value());
-            context.display_list_recorder().fill_rect_with_rounded_corners(thumb_device_rect.to_type<int>(), color, thumb_corner_radius, thumb_corner_radius, thumb_corner_radius, thumb_corner_radius);
+            context.display_list_recorder().fill_rect_with_rounded_corners(thumb_device_rect.to_type<int>(), color, thumb_corner_radius);
         }
         if (auto thumb_rect = scroll_thumb_rect(ScrollDirection::Vertical); thumb_rect.has_value()) {
             auto thumb_device_rect = context.enclosing_device_rect(thumb_rect.value());
-            context.display_list_recorder().fill_rect_with_rounded_corners(thumb_device_rect.to_type<int>(), color, thumb_corner_radius, thumb_corner_radius, thumb_corner_radius, thumb_corner_radius);
+            context.display_list_recorder().fill_rect_with_rounded_corners(thumb_device_rect.to_type<int>(), color, thumb_corner_radius);
         }
     }