auto merge of #5067 : servo/servo/counters, r=SimonSapin

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.

Moved from #4544, because Critic.

Fixes #4544.

components/layout/display_list_builder.rs

@@ -885,6 +885,7 @@ impl FragmentDisplayListBuilding for Fragment {
                 }
             }
             SpecificFragmentInfo::Generic |
+            SpecificFragmentInfo::GeneratedContent(..) |
             SpecificFragmentInfo::Iframe(..) |
             SpecificFragmentInfo::Table |
             SpecificFragmentInfo::TableCell |