layout: Implement support for font fallback (#31254)

This adds an initial implementation of font fallback, on part with the
one used in legacy layout. There are still issues. For instance, font
matching is done per unicode character rather than based on graphemes or
the shape first approach of Chrome. The idea is that these changes can
be made later.

components/layout_2020/flow/construct.rs

@@ -67,6 +67,7 @@ impl BlockFormattingContext {
 
         let ifc = InlineFormattingContext {
             inline_level_boxes,
+            font_metrics: Vec::new(),
             text_decoration_line,
             has_first_formatted_line: true,
             contains_floats: false,
@@ -433,13 +434,12 @@ where
             _ => {},
         }
 
-        inlines.push(ArcRefCell::new(InlineLevelBox::TextRun(TextRun {
-            base_fragment_info: info.into(),
-            parent_style: Arc::clone(&info.style),
-            text: output,
+        inlines.push(ArcRefCell::new(InlineLevelBox::TextRun(TextRun::new(
+            info.into(),
+            Arc::clone(&info.style),
+            output,
             has_uncollapsible_content,
-            shaped_text: None,
-        })));
+        ))));
     }
 }
 
@@ -606,6 +606,7 @@ where
                 is_first_fragment: true,
                 is_last_fragment: false,
                 children: vec![],
+                default_font_index: None,
             });
 
             if is_list_item {
@@ -670,6 +671,7 @@ where
                         // are obviously not the last fragment.
                         is_last_fragment: false,
                         children: std::mem::take(&mut ongoing.children),
+                        default_font_index: None,
                     };
                     ongoing.is_first_fragment = false;
                     fragmented