CSS Grid: percentage sizing fixes (#34948)

This applies some fixes for CSS Grid percentage sizing. These fixes are
mostly within Taffy, but there are some changes in Servo to allow it to
communicate whether an item is replaced to Taffy.

It also updates Taffy to v0.8.0. Taffy has switched to a tagged pointer
representation of length/size styles. Much of the diff here is updating
Servo's type conversion code to use the new representation.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes OR

---------

Signed-off-by: Nico Burns <nico@nicoburns.com>

components/layout/taffy/mod.rs

@@ -19,7 +19,9 @@ use crate::construct_modern::{ModernContainerBuilder, ModernItemKind};
 use crate::context::LayoutContext;
 use crate::dom::LayoutBox;
 use crate::dom_traversal::{NodeAndStyleInfo, NonReplacedContents};
-use crate::formatting_contexts::IndependentFormattingContext;
+use crate::formatting_contexts::{
+    IndependentFormattingContext, IndependentFormattingContextContents,
+};
 use crate::fragment_tree::Fragment;
 use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
 
@@ -166,6 +168,16 @@ impl TaffyItemBox {
                 .repair_style(context, node, new_style),
         }
     }
+
+    fn is_in_flow_replaced(&self) -> bool {
+        match &self.taffy_level_box {
+            TaffyItemBoxInner::InFlowBox(fc) => match fc.contents {
+                IndependentFormattingContextContents::NonReplaced(_) => false,
+                IndependentFormattingContextContents::Replaced(_) => true,
+            },
+            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(_) => false,
+        }
+    }
 }
 
 /// Details from Taffy grid layout that will be stored