layout: Make IndependentFormattingContext::contents private (again) (#38350)

This was done in #24871, but after some refactorings it became public.
This makes it private again. As said in
b2b3ea992c:

> Privacy forces the rest of the code to go through methods
> rather than matching on the enum,
> reducing accidental layout-mode-specific behavior.

It also avoids the risk of accidentally calling `layout()` on the inner
layout-mode-specific struct, bypassing caching.

Testing: Not needed (no behavior change)

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/layout/construct_modern.rs

@@ -69,10 +69,10 @@ impl<'dom> ModernContainerJob<'dom> {
                     BlockContainer::InlineFormattingContext(inline_formatting_context),
                 );
                 let info: &NodeAndStyleInfo = anonymous_info;
-                let formatting_context = IndependentFormattingContext {
-                    base: LayoutBoxBase::new(info.into(), info.style.clone()),
-                    contents: IndependentFormattingContextContents::Flow(block_formatting_context),
-                };
+                let formatting_context = IndependentFormattingContext::new(
+                    LayoutBoxBase::new(info.into(), info.style.clone()),
+                    IndependentFormattingContextContents::Flow(block_formatting_context),
+                );
 
                 Some(ModernItem {
                     kind: ModernItemKind::InFlow(formatting_context),