[heap] Add third-party heap guard in String::LooksValid().

Bug: v8:9533
Change-Id: Id888929b45a1519f4b4a4bb73159816520461841
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2534478
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71150}

src/objects/string.cc

@@ -439,6 +439,7 @@ bool String::LooksValid() {
   // TODO(leszeks): Maybe remove this check entirely, Heap::Contains uses
   // basically the same logic as the way we access the heap in the first place.
   // RO_SPACE objects should always be valid.
+  if (V8_ENABLE_THIRD_PARTY_HEAP_BOOL) return true;
   if (ReadOnlyHeap::Contains(*this)) return true;
   BasicMemoryChunk* chunk = BasicMemoryChunk::FromHeapObject(*this);
   if (chunk->heap() == nullptr) return false;