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/flow/float.rs

@@ -891,7 +891,7 @@ impl FloatBox {
     /// the float containing block formatting context. A later step adjusts the position
     /// to be relative to the containing block.
     pub fn layout(
-        &mut self,
+        &self,
         layout_context: &LayoutContext,
         positioning_context: &mut PositioningContext,
         containing_block: &ContainingBlock,