[heap] Cleanup mark bit usage.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 980e4b3d373aeba23d242eec949edd1001a912b4..e3c637ac89bb347b892f1321639d30d86c0d79c5 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -1417,8 +1417,9 @@ typedef StringTableCleaner<true> ExternalStringTableCleaner;
 class MarkCompactWeakObjectRetainer : public WeakObjectRetainer {
  public:
   virtual Object* RetainAs(Object* object) {
-    if (Marking::IsBlackOrGrey(
-            Marking::MarkBitFrom(HeapObject::cast(object)))) {
+    MarkBit mark_bit = Marking::MarkBitFrom(HeapObject::cast(object));
+    DCHECK(!Marking::IsGrey(mark_bit));
+    if (Marking::IsBlack(mark_bit)) {
       return object;
     } else if (object->IsAllocationSite() &&
                !(AllocationSite::cast(object)->IsZombie())) {
@@ -2327,8 +2328,8 @@ void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map) {
 
   // Follow back pointer, check whether we are dealing with a map transition
   // from a live map to a dead path and in case clear transitions of parent.
-  DCHECK(!Marking::IsBlackOrGrey(Marking::MarkBitFrom(map)));
-  bool parent_is_alive = Marking::IsBlackOrGrey(Marking::MarkBitFrom(parent));
+  DCHECK(!Marking::IsGrey(Marking::MarkBitFrom(map)));
+  bool parent_is_alive = Marking::IsBlack(Marking::MarkBitFrom(parent));
   if (parent_is_alive) {
     ClearMapTransitions(parent, map);
   }
@@ -2338,7 +2339,8 @@ void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map) {
 // Clear a possible back pointer in case the transition leads to a dead map.
 // Return true in case a back pointer has been cleared and false otherwise.
 bool MarkCompactCollector::ClearMapBackPointer(Map* target) {
-  if (Marking::IsBlackOrGrey(Marking::MarkBitFrom(target))) return false;
+  DCHECK(!Marking::IsGrey(Marking::MarkBitFrom(target)));
+  if (Marking::IsBlack(Marking::MarkBitFrom(target))) return false;
   target->SetBackPointer(heap_->undefined_value(), SKIP_WRITE_BARRIER);
   return true;
 }