Don't UnshiftGrey any black objects

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1128683006

Cr-Commit-Position: refs/heads/master@{#28582}

src/heap/incremental-marking.cc

@@ -188,7 +188,12 @@ class IncrementalMarkingMarkingVisitor
       } while (scan_until_end && start_offset < object_size);
       chunk->set_progress_bar(start_offset);
       if (start_offset < object_size) {
-        heap->mark_compact_collector()->marking_deque()->UnshiftGrey(object);
+        if (Marking::IsGrey(Marking::MarkBitFrom(object))) {
+          heap->mark_compact_collector()->marking_deque()->UnshiftGrey(object);
+        } else {
+          DCHECK(Marking::IsBlack(Marking::MarkBitFrom(object)));
+          heap->mark_compact_collector()->marking_deque()->UnshiftBlack(object);
+        }
         heap->incremental_marking()->NotifyIncompleteScanOfObject(
             object_size - (start_offset - already_scanned_offset));
       }

src/heap/mark-compact.h

@@ -242,6 +242,7 @@ class MarkingDeque {
 
   INLINE(void UnshiftGrey(HeapObject* object)) {
     DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsGrey(Marking::MarkBitFrom(object)));
     if (IsFull()) {
       SetOverflowed();
     } else {
@@ -250,6 +251,19 @@ class MarkingDeque {
     }
   }
 
+  INLINE(void UnshiftBlack(HeapObject* object)) {
+    DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsBlack(Marking::MarkBitFrom(object)));
+    if (IsFull()) {
+      Marking::BlackToGrey(object);
+      MemoryChunk::IncrementLiveBytesFromGC(object->address(), -object->Size());
+      SetOverflowed();
+    } else {
+      bottom_ = ((bottom_ - 1) & mask_);
+      array_[bottom_] = object;
+    }
+  }
+
   HeapObject** array() { return array_; }
   int bottom() { return bottom_; }
   int top() { return top_; }