[heap] Remove ObjectMarking::BlackToWhite and ObjectMarking::GreyToWhite.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2734823003
Cr-Commit-Position: refs/heads/master@{#43607}

src/heap/mark-compact.h

@@ -77,20 +77,6 @@ class ObjectMarking : public AllStatic {
     Marking::MarkWhite(MarkBitFrom<mode>(obj));
   }
 
-  template <MarkingMode mode = MarkingMode::FULL>
-  V8_INLINE static void BlackToWhite(HeapObject* obj) {
-    DCHECK(IsBlack<mode>(obj));
-    MarkBit markbit = MarkBitFrom<mode>(obj);
-    Marking::BlackToWhite(markbit);
-    MemoryChunk::IncrementLiveBytes<mode>(obj, -obj->Size());
-  }
-
-  template <MarkingMode mode = MarkingMode::FULL>
-  V8_INLINE static void GreyToWhite(HeapObject* obj) {
-    DCHECK(IsGrey<mode>(obj));
-    Marking::GreyToWhite(MarkBitFrom<mode>(obj));
-  }
-
   template <MarkingMode mode = MarkingMode::FULL>
   V8_INLINE static void BlackToGrey(HeapObject* obj) {
     DCHECK(IsBlack<mode>(obj));

src/heap/marking.h

@@ -312,7 +312,6 @@ class Marking : public AllStatic {
   INLINE(static void GreyToWhite(MarkBit markbit)) {
     DCHECK(IsGrey(markbit));
     markbit.Clear();
-    markbit.Next().Clear();
   }
 
   INLINE(static void BlackToGrey(MarkBit markbit)) {