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/layout_task.rs

@@ -840,6 +840,14 @@ impl LayoutTask {
             layout_debug::begin_trace(layout_root.clone());
         }
 
+        // Resolve generated content.
+        profile(TimeProfilerCategory::LayoutGeneratedContent,
+                self.profiler_metadata(data),
+                self.time_profiler_chan.clone(),
+                || {
+                    sequential::resolve_generated_content(&mut layout_root, &shared_layout_context)
+                });
+
         // Perform the primary layout passes over the flow tree to compute the locations of all
         // the boxes.
         profile(TimeProfilerCategory::LayoutMain,