Bug 1868048 - StyleSheetList doesn't need to observe all the mutations, r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D195372

dom/base/StyleSheetList.cpp

@@ -34,6 +34,7 @@ void StyleSheetList::NodeWillBeDestroyed(nsINode* aNode) {
 
 StyleSheetList::StyleSheetList(DocumentOrShadowRoot& aScope)
     : mDocumentOrShadowRoot(&aScope) {
+  SetEnabledCallbacks(nsIMutationObserver::kNodeWillBeDestroyed);
   mDocumentOrShadowRoot->AsNode().AddMutationObserver(this);
 }