layout: Implement ordered lists, CSS counters, and quotes per CSS 2.1

§ 12.3-12.5.

Only simple alphabetic and numeric counter styles are supported. (This
is most of them though.)

Although this PR adds a sequential pass to layout, I verified that on
pages that contain a reasonable number of ordered lists (Reddit
`/r/rust`), the time spent in generated content resolution is dwarfed by
the time spent in the parallelizable parts of layout. So I don't expect
this to negatively affect our parallelism expect perhaps in pathological
cases.

components/layout/table_caption.rs

@@ -7,10 +7,9 @@
 #![deny(unsafe_blocks)]
 
 use block::BlockFlow;
-use construct::FlowConstructor;
 use context::LayoutContext;
 use flow::{FlowClass, Flow};
-use fragment::FragmentBorderBoxIterator;
+use fragment::{Fragment, FragmentBorderBoxIterator};
 use wrapper::ThreadSafeLayoutNode;
 
 use geom::{Point2D, Rect};
@@ -26,11 +25,10 @@ pub struct TableCaptionFlow {
 }
 
 impl TableCaptionFlow {
-    pub fn from_node(constructor: &mut FlowConstructor,
-                     node: &ThreadSafeLayoutNode)
-                     -> TableCaptionFlow {
+    pub fn from_node_and_fragment(node: &ThreadSafeLayoutNode, fragment: Fragment)
+                                  -> TableCaptionFlow {
         TableCaptionFlow {
-            block_flow: BlockFlow::from_node(constructor, node)
+            block_flow: BlockFlow::from_node_and_fragment(node, fragment)
         }
     }
 }
@@ -96,6 +94,10 @@ impl Flow for TableCaptionFlow {
                                              stacking_context_position: &Point2D<Au>) {
         self.block_flow.iterate_through_fragment_border_boxes(iterator, stacking_context_position)
     }
+
+    fn mutate_fragments(&mut self, mutator: &mut FnMut(&mut Fragment)) {
+        self.block_flow.mutate_fragments(mutator)
+    }
 }
 
 impl fmt::Debug for TableCaptionFlow {