[heap] Skip StringTable iteration if skipping old

Since the StringTable can only contain old strings, skip iterating it
when SkipRoot::kOldGeneration is set.

Bug: chromium:1115132, chromium:1115100
Change-Id: I6d065a0ea7f3142c5d474eb0919e801e13976f6c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2351664
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69353}

src/heap/heap.cc

@@ -4396,9 +4396,10 @@ void Heap::set_builtin(int index, Code builtin) {
 void Heap::IterateWeakRoots(RootVisitor* v, base::EnumSet<SkipRoot> options) {
   DCHECK(!options.contains(SkipRoot::kWeak));
 
-  if (!options.contains(SkipRoot::kUnserializable)) {
+  if (!options.contains(SkipRoot::kOldGeneration) &&
+      !options.contains(SkipRoot::kUnserializable)) {
     // Do not visit for serialization, since the string table is custom
-    // serialized.
+    // serialized. Also do not visit if we are skipping old generation.
     isolate()->string_table()->IterateElements(v);
   }
   v->Synchronize(VisitorSynchronization::kStringTable);