[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..88128145e4b407ae7dfef2871a2886d1175774b4 100644
--- a/src/heap/marking.h
+++ b/src/heap/marking.h
@@ -298,14 +298,9 @@ 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 MarkWhite(MarkBit markbit)) {
+    markbit.Clear();
+    markbit.Next().Clear();
   }
 
   INLINE(static void BlackToWhite(MarkBit markbit)) {