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/layout_box_base.rs

@@ -27,7 +27,6 @@ use crate::{ConstraintSpace, ContainingBlockSize};
 #[derive(MallocSizeOf)]
 pub(crate) struct LayoutBoxBase {
     pub base_fragment_info: BaseFragmentInfo,
-    #[conditional_malloc_size_of]
     pub style: Arc<ComputedValues>,
     pub cached_inline_content_size:
         AtomicRefCell<Option<Box<(SizeConstraint, InlineContentSizesResult)>>>,