[heap] Do not clear mark bits of left trimmed old object start.

src/heap/marking.h

Index: src/heap/marking.h
diff --git a/src/heap/marking.h b/src/heap/marking.h
index b8396b7f0b243f9036956761446571a2166dd727..3c217454892ecf87c2ec92a20b5b1d76d298b0f1 100644
--- a/src/heap/marking.h
+++ b/src/heap/marking.h
@@ -303,6 +303,13 @@ class Marking : public AllStatic {
     markbit.Next().Clear();
   }
 
+  INLINE(static void MarkGrey(MarkBit markbit)) { markbit.Set(); }
+
+  INLINE(static void MarkBlack(MarkBit markbit)) {
+    markbit.Set();
+    markbit.Next().Set();
+  }
+
   INLINE(static void BlackToGrey(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Next().Clear();