layout: Share styles to inline box children via SharedInlineStyles (#36896)

`TextRun`s use their parent style to render. Previously, these styles
were cloned and stored directly in the box tree `TextRun` and resulting
`TextFragment`s. This presents a problem for incremental layout.
Wrapping the style in another layer of shared ownership and mutability
will allow updating all `TextFragment`s during repaint-only incremental
layout by simply updating the box tree styles of the original text
parents.

This adds a new set of borrows when accessing text styles, but also
makes it so that during box tree block construction
`InlineFormattingContext`s are created lazily and now
`InlineFormattingContextBuilder::finish` consumes the builder, making
the API make a bit more sense. This should also improve performance of
box tree block construction slightly.

Testing: This should not change observable behavior and thus is covered
by existing WPT tests.

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

components/layout/taffy/mod.rs

@@ -24,7 +24,6 @@ use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
 #[derive(Debug, MallocSizeOf)]
 pub(crate) struct TaffyContainer {
     children: Vec<ArcRefCell<TaffyItemBox>>,
-    #[conditional_malloc_size_of]
     style: Arc<ComputedValues>,
 }
 
@@ -76,7 +75,6 @@ pub(crate) struct TaffyItemBox {
     pub(crate) taffy_layout: taffy::Layout,
     pub(crate) child_fragments: Vec<Fragment>,
     pub(crate) positioning_context: PositioningContext,
-    #[conditional_malloc_size_of]
     pub(crate) style: Arc<ComputedValues>,
     pub(crate) taffy_level_box: TaffyItemBoxInner,
 }