cppgc: Provide API to test if object is marked or freed

This is needed to support minor gc tests on the Blink side.

Bug: chromium:1029379
Change-Id: Ic7ce1ae8e626b87738d76663ceecf7198e425d18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3437050Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78943}

include/cppgc/testing.h

@@ -93,6 +93,8 @@ class V8_EXPORT StandaloneTestingHeap final {
   HeapHandle& heap_handle_;
 };
 
+V8_EXPORT bool IsHeapObjectOld(void*);
+
 }  // namespace testing
 }  // namespace cppgc
 

src/heap/cppgc/testing.cc

@@ -54,5 +54,13 @@ void StandaloneTestingHeap::ForceCompactionForNextGarbageCollection() {
       .EnableForNextGCForTesting();
 }
 
+bool IsHeapObjectOld(void* object) {
+#if defined(CPPGC_YOUNG_GENERATION)
+  return internal::HeapObjectHeader::FromObject(object).IsMarked();
+#else
+  return true;
+#endif
+}
+
 }  // namespace testing
 }  // namespace cppgc