Use a RwLock to cache inline_content_sizes() (#34232)

In order to support size keywords in block layout, we may need to call
`inline_content_sizes()` in order to compute the min/max-content sizes.
But this required a mutable reference in order the update the cache,
and in various places we already had mutable references.

So this switches the cache into a RwLock to avoid needing mutable refs.
Note OnceCell wouldn't work because it's not thread-safe.

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

components/layout_2020/flexbox/construct.rs

@@ -171,7 +171,7 @@ where
                     let non_replaced = NonReplacedFormattingContext {
                         base_fragment_info: info.into(),
                         style: info.style.clone(),
-                        content_sizes_result: None,
+                        content_sizes_result: Default::default(),
                         contents: NonReplacedFormattingContextContents::Flow(
                             block_formatting_context,
                         ),