Bug 1867789 - Use the reference to a pointer to avoid recreating a new ImplicitNamedAreas in each iteration. r=emilio

I forgot to use a reference when doing the refactoring. We should use a
reference to a pointer because we rely on the information from the previous
iteration, in the private function, `AddImplicitNamedAreasInternal()`.

Note: I don't have a test for this. This bug is from a real website:
https://www.oddbird.net/tools/, and it's not easy to reduce the
testcase, so we may need a QA to verify this by the real website.

Differential Revision: https://phabricator.services.mozilla.com/D195388

layout/generic/nsGridContainerFrame.cpp

@@ -3959,7 +3959,8 @@ nsContainerFrame* NS_NewGridContainerFrame(PresShell* aPresShell,
 }
 
 void nsGridContainerFrame::AddImplicitNamedAreasInternal(
-    LineNameList& aNameList, nsGridContainerFrame::ImplicitNamedAreas* aAreas) {
+    LineNameList& aNameList,
+    nsGridContainerFrame::ImplicitNamedAreas*& aAreas) {
   for (const auto& nameIdent : aNameList.AsSpan()) {
     nsAtom* name = nameIdent.AsAtom();
     uint32_t indexOfSuffix;

layout/generic/nsGridContainerFrame.h

@@ -513,7 +513,7 @@ class nsGridContainerFrame final : public nsContainerFrame,
 
   // The internal implementation for AddImplicitNamedAreas().
   void AddImplicitNamedAreasInternal(LineNameList& aNameList,
-                                     ImplicitNamedAreas* aAreas);
+                                     ImplicitNamedAreas*& aAreas);
 
   /**
    * Cached values to optimize GetMinISize/GetPrefISize.