Issue #1757 - Reinstate "dom.details_element.enabled" preference

The removal of this preference was botched, all other surrounding plumbing changes appear to be working okay. The ability to use prefs to control this stylesheet might be useful in the future, so perhaps this is one of those "if it ain't broke, don't fix it" bugs where leaving well enough alone in the first place would have been the best choice.

layout/generic/nsContainerFrame.cpp

@@ -1891,10 +1891,12 @@ nsContainerFrame::RenumberFrameAndDescendants(int32_t* aOrdinal,
   }
 
   // Do not renumber list for summary elements.
-  HTMLSummaryElement* summary =
-    HTMLSummaryElement::FromContent(kid->GetContent());
-  if (summary && summary->IsMainSummary()) {
-    return false;
+  if (HTMLDetailsElement::IsDetailsEnabled()) {
+    HTMLSummaryElement* summary =
+      HTMLSummaryElement::FromContent(kid->GetContent());
+    if (summary && summary->IsMainSummary()) {
+      return false;
+    }
   }
 
   bool kidRenumberedABullet = false;