Bug 1422931 - Part 2: Make webcomponents preference per-doc. r=smaug

This is to fix the case where preference is restore to false when a
testcas ends, but nsDocument::DeleteShell is called afterwards. So, we
make the preference per-doc and once it is enabled for a document, it
stays enabled.

dom/html/HTMLSlotElement.cpp

@@ -16,7 +16,7 @@ NS_NewHTMLSlotElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
                       mozilla::dom::FromParser aFromParser)
 {
   RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
-  if (nsContentUtils::IsWebComponentsEnabled()) {
+  if (nsDocument::IsWebComponentsEnabled(nodeInfo->GetDocument())) {
     already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
     return new mozilla::dom::HTMLSlotElement(nodeInfoArg);
   }