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

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 86d0b9616b5048822766280694daa4269b23e0a7..42aa243a032c31b779f2f67472e65004f9025073 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -67,7 +67,8 @@ class ObjectMarking : public AllStatic {
   }
 
   V8_INLINE static void ClearMarkBit(HeapObject* obj) {

[ulan, 2017/02/28 15:43:47]

If object is black at the callsites, can we just used BlackToWhite(obj) directly
and remove this function?

It is not clear from the name that the object must be black.

[Hannes Payer (out of office), 2017/03/01 13:49:18]

No we cannot, since the object can also be grey (when marking aborts).

-    Marking::MarkWhite(MarkBitFrom(obj));
+    DCHECK(IsBlack(obj));
+    Marking::BlackToWhite(MarkBitFrom(obj));
   }
 
   V8_INLINE static void BlackToWhite(HeapObject* obj) {