Don't UnshiftGrey any black objects

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 95f522bd991fe83fadb9baa8445fd7768b736f6f..2184c72d31a3b091a11d5336d413662d9e897b60 100644
--- a/src/heap/incremental-marking.cc
+++ b/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) {

[Hannes Payer (out of office), 2015/05/15 15:26:06]

As discussed offline, let's figure out the reason first why the invariant *here
should be just grey object* does not hold.

-        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));
       }