Add checks to the marking queue

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index e26e06c24fa6c1c946a69dffb098d5f6e34d73f4..361f3a6377392022e69931832f2ee8ec6962d91d 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -168,6 +168,8 @@ class MarkingDeque {
   // heap.
   INLINE(void PushBlack(HeapObject* object)) {
     DCHECK(object->IsHeapObject());
+    // TODO(jochen): Remove again before we branch for 4.2.
+    CHECK(object->IsHeapObject() && object->map()->IsMap());
     if (IsFull()) {
       Marking::BlackToGrey(object);
       MemoryChunk::IncrementLiveBytesFromGC(object->address(), -object->Size());
@@ -180,6 +182,8 @@ class MarkingDeque {
 
   INLINE(void PushGrey(HeapObject* object)) {
     DCHECK(object->IsHeapObject());
+    // TODO(jochen): Remove again before we branch for 4.2.
+    CHECK(object->IsHeapObject() && object->map()->IsMap());
     if (IsFull()) {
       SetOverflowed();
     } else {