[heap] Make the current GCCallbackFlags are part of {Heap}.

test/cctest/cctest.h

Index: test/cctest/cctest.h
diff --git a/test/cctest/cctest.h b/test/cctest/cctest.h
index 40fb2390cd61bd56c35844f6e86abc9d699b4856..6c95764f21a0f8ae07ece34e18a8c93ddc28eb57 100644
--- a/test/cctest/cctest.h
+++ b/test/cctest/cctest.h
@@ -546,7 +546,7 @@ static inline void SimulateIncrementalMarking(i::Heap* heap,
   }
   CHECK(marking->IsMarking() || marking->IsStopped());
   if (marking->IsStopped()) {
-    marking->Start(i::Heap::kNoGCFlags);
+    heap->StartIncrementalMarking(i::Heap::kNoGCFlags);
   }
   CHECK(marking->IsMarking());
   if (!force_completion) return;