Bug 1642949 - Replace uses of RemoveElementAt by RemoveLastElement/PopLastElement where possible. r=necko-reviewers,froydnj

Differential Revision: https://phabricator.services.mozilla.com/D78027

layout/style/ServoStyleSet.cpp

@@ -981,10 +981,7 @@ bool ServoStyleSet::EnsureUniqueInnerOnCSSSheets() {
   });
 
   while (!queue.IsEmpty()) {
-    uint32_t idx = queue.Length() - 1;
-    auto* sheet = queue[idx].first;
-    SheetOwner owner = queue[idx].second;
-    queue.RemoveElementAt(idx);
+    auto [sheet, owner] = queue.PopLastElement();
 
     // Only call EnsureUniqueInner for complete sheets. If we do call it on
     // incomplete sheets, we'll cause problems when the sheet is actually