[heap] Remove Marking::MarkBlack and some minor cleanups.

src/heap/marking.h

Index: src/heap/marking.h
diff --git a/src/heap/marking.h b/src/heap/marking.h
index 1248e59fe2cc142abbce46e010348287e644b5d5..2c0c32aeff4f3331c3e13055c6827c1dc8ee99d2 100644
--- a/src/heap/marking.h
+++ b/src/heap/marking.h
@@ -298,16 +298,6 @@ class Marking : public AllStatic {
   // objects.
   INLINE(static bool IsBlackOrGrey(MarkBit mark_bit)) { return mark_bit.Get(); }
 
-  INLINE(static void MarkBlack(MarkBit mark_bit)) {
-    mark_bit.Set();
-    mark_bit.Next().Set();
-  }
-
-  INLINE(static void MarkWhite(MarkBit mark_bit)) {
-    mark_bit.Clear();
-    mark_bit.Next().Clear();
-  }
-
   INLINE(static void BlackToWhite(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Clear();