[heap] Unify MarkingDeque push and unshift operations.

src/heap/mark-compact-inl.h

Index: src/heap/mark-compact-inl.h
diff --git a/src/heap/mark-compact-inl.h b/src/heap/mark-compact-inl.h
index 2d2feb29e5b169828569497bf7213460da0e0ab3..b5f58462dad24bd1d1400ec7eafd366cd05e2ad8 100644
--- a/src/heap/mark-compact-inl.h
+++ b/src/heap/mark-compact-inl.h
@@ -24,7 +24,8 @@ void MarkCompactCollector::SetFlags(int flags) {
 
 
 void MarkCompactCollector::PushBlack(HeapObject* obj) {
-  if (marking_deque_.PushBlack(obj)) {
+  DCHECK(Marking::IsBlack(Marking::MarkBitFrom(obj)));
+  if (marking_deque_.Push(obj)) {
     MemoryChunk::IncrementLiveBytesFromGC(obj, obj->Size());
   } else {
     Marking::BlackToGrey(obj);
@@ -33,7 +34,8 @@ void MarkCompactCollector::PushBlack(HeapObject* obj) {
 
 
 void MarkCompactCollector::UnshiftBlack(HeapObject* obj) {
-  if (!marking_deque_.UnshiftBlack(obj)) {
+  DCHECK(Marking::IsBlack(Marking::MarkBitFrom(obj)));
+  if (!marking_deque_.Unshift(obj)) {
     MemoryChunk::IncrementLiveBytesFromGC(obj, -obj->Size());
     Marking::BlackToGrey(obj);
   }