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_rowgroup.rs

@@ -7,7 +7,6 @@
 #![deny(unsafe_blocks)]
 
 use block::{BlockFlow, ISizeAndMarginsComputer, MarginsMayCollapseFlag};
-use construct::FlowConstructor;
 use context::LayoutContext;
 use flow::{FlowClass, Flow};
 use fragment::{Fragment, FragmentBorderBoxIterator};
@@ -45,15 +44,6 @@ impl TableRowGroupFlow {
         }
     }
 
-    pub fn from_node(constructor: &mut FlowConstructor, node: &ThreadSafeLayoutNode)
-                     -> TableRowGroupFlow {
-        TableRowGroupFlow {
-            block_flow: BlockFlow::from_node(constructor, node),
-            column_intrinsic_inline_sizes: Vec::new(),
-            column_computed_inline_sizes: Vec::new(),
-        }
-    }
-
     pub fn fragment<'a>(&'a mut self) -> &'a Fragment {
         &self.block_flow.fragment
     }
@@ -165,6 +155,10 @@ impl Flow for TableRowGroupFlow {
                                              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 TableRowGroupFlow {