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/table/construct.rs

@@ -137,7 +137,7 @@ impl Table {
         IndependentFormattingContext::NonReplaced(NonReplacedFormattingContext {
             base_fragment_info: (&anonymous_info).into(),
             style: grid_and_wrapper_style,
-            content_sizes_result: None,
+            content_sizes_result: Default::default(),
             contents: NonReplacedFormattingContextContents::Table(table),
         })
     }
@@ -858,7 +858,7 @@ where
                         context: ArcRefCell::new(NonReplacedFormattingContext {
                             style: info.style.clone(),
                             base_fragment_info: info.into(),
-                            content_sizes_result: None,
+                            content_sizes_result: Default::default(),
                             contents,
                         }),
                     };