cppgc: young-gen: Fix generational barrier when value is not present

params.start was not set when marker was present, however the barrier
relies on params.start.

Bug: chromium:1029379
Change-Id: I12cfbdbdec63518cfc0d0d174085ea8aefb3a375
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3605822Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80207}

src/heap/cppgc/write-barrier.cc

@@ -188,16 +188,13 @@ bool WriteBarrierTypeForNonCagedHeapPolicy::IsMarking(HeapHandle& heap_handle) {
 bool WriteBarrierTypeForCagedHeapPolicy::IsMarking(
     const HeapHandle& heap_handle, WriteBarrier::Params& params) {
   const auto& heap_base = internal::HeapBase::From(heap_handle);
-  if (const MarkerBase* marker = heap_base.marker()) {
-    return marker->IsMarking();
-  }
+  const bool is_marking = heap_base.marker() && heap_base.marker()->IsMarking();
   // Also set caged heap start here to avoid another call immediately after
   // checking IsMarking().
 #if defined(CPPGC_YOUNG_GENERATION)
-  params.start =
-      reinterpret_cast<uintptr_t>(&heap_base.caged_heap().local_data());
+  params.start = reinterpret_cast<uintptr_t>(heap_base.caged_heap().base());
 #endif  // !CPPGC_YOUNG_GENERATION
-  return false;
+  return is_marking;
 }
 
 #endif  // CPPGC_CAGED_HEAP