[heap] Add comment to Factory::InternalizedStringMapForString

Better explain why internalized strings have to be in old space. The
scavenger doesn't need to iterate and update references in the string
table and the stub cache.

Change-Id: I93c3e0b743f85fbf4de2ad877f3667abb2e0ae53
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2949101
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75052}

src/heap/factory.cc

@@ -830,8 +830,8 @@ MaybeHandle<Map> GetInternalizedStringMap(Factory* f, Handle<String> string) {
 
 MaybeHandle<Map> Factory::InternalizedStringMapForString(
     Handle<String> string) {
-  // If the string is in the young generation, it cannot be used as
-  // internalized.
+  // Do not internalize young strings: This allows us to ignore both string
+  // table and stub cache on scavenges.
   if (Heap::InYoungGeneration(*string)) return MaybeHandle<Map>();
   return GetInternalizedStringMap(this, string);
 }