[heap] Make non-atomic markbit operations consistent with atomic ones.

src/heap/incremental-marking.h

Index: src/heap/incremental-marking.h
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index 200fbf39c48a8114bc3faf271e90a232ab8ee930..d3ce3c8c84633363ec3cfbc3e7abdd0cdcf24880 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -45,7 +45,6 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   }
 
   void MarkBlack(HeapObject* object, int size);
-  void MarkGrey(HeapObject* object);
 
   // Transfers mark bits without requiring proper object headers.
   void TransferMark(Heap* heap, HeapObject* from, HeapObject* to);
@@ -61,9 +60,15 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
 
     DCHECK(ObjectMarking::IsWhite<access_mode>(to, marking_state(to)));
     if (ObjectMarking::IsGrey<access_mode>(from, marking_state(from))) {
-      ObjectMarking::WhiteToGrey<access_mode>(to, marking_state(to));
+      bool success =
+          ObjectMarking::WhiteToGrey<access_mode>(to, marking_state(to));
+      DCHECK(success);
+      USE(success);
     } else if (ObjectMarking::IsBlack<access_mode>(from, marking_state(from))) {
-      ObjectMarking::WhiteToBlack<access_mode>(to, marking_state(to));
+      bool success =
+          ObjectMarking::WhiteToBlack<access_mode>(to, marking_state(to));
+      DCHECK(success);
+      USE(success);
     }
   }
 
@@ -189,7 +194,7 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   void RecordCodeTargetPatch(Code* host, Address pc, HeapObject* value);
   void RecordCodeTargetPatch(Address pc, HeapObject* value);
 
-  void WhiteToGreyAndPush(HeapObject* obj);
+  bool WhiteToGreyAndPush(HeapObject* obj);
 
   inline void SetOldSpacePageFlags(MemoryChunk* chunk) {
     SetOldSpacePageFlags(chunk, IsMarking(), IsCompacting());