layout: Cache content block size contributions (#33964)

This is the first part of caching intermediary layout during flexbox
layout. A later change will try to reuse these layouts, when possible,
for actual item layout and re-layout due to stretching.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/flexbox/construct.rs

@@ -181,6 +181,7 @@ where
                         independent_formatting_context: IndependentFormattingContext::NonReplaced(
                             non_replaced,
                         ),
+                        cached_layout: Default::default(),
                     })))
                 },
                 FlexLevelJob::Element {
@@ -212,6 +213,7 @@ where
                                 contents,
                                 self.text_decoration_line,
                             ),
+                            cached_layout: Default::default(),
                         }))
                     };
                     box_slot.set(LayoutBox::FlexLevel(box_.clone()));