Clear mark bits while sweeping and not explicitly.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 24adeca7924248779aa0122f1c429ee3e6be1d72..ce4738ac5649182ef74fa0f40ea00ccd757b0e4c 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -440,6 +440,12 @@ class MarkCompactCollector {
     PRECISE
   };
 
+#ifdef DEBUG
+  void VerifyMarkbitsAreClean();
+  static void VerifyMarkbitsAreClean(PagedSpace* space);
+  static void VerifyMarkbitsAreClean(NewSpace* space);
+#endif
+
   // Sweep a single page from the given space conservatively.
   // Return a number of reclaimed bytes.
   static intptr_t SweepConservatively(PagedSpace* space, Page* p);