[heap] Make typed slot set state and operations atomic.

src/heap/remembered-set.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_REMEMBERED_SET_H
 #define V8_REMEMBERED_SET_H
 
 #include "src/assembler.h"
 #include "src/heap/heap.h"
 #include "src/heap/slot-set.h"
@@ skipping 345 matching lines @@
         }
         return REMOVE_SLOT;
       }
       case EMBEDDED_OBJECT_SLOT: {
         RelocInfo rinfo(isolate, addr, RelocInfo::EMBEDDED_OBJECT, 0, NULL);
         return UpdateEmbeddedPointer(&rinfo, callback);
       }
       case OBJECT_SLOT: {
         return callback(reinterpret_cast<Object**>(addr));
       }
-      case NUMBER_OF_SLOT_TYPES:
+      case CLEARED_SLOT:
         break;
     }
     UNREACHABLE();
     return REMOVE_SLOT;
   }
 };
 
 inline SlotType SlotTypeForRelocInfoMode(RelocInfo::Mode rmode) {
   if (RelocInfo::IsCodeTarget(rmode)) {
     return CODE_TARGET_SLOT;
   } else if (RelocInfo::IsCell(rmode)) {
     return CELL_TARGET_SLOT;
   } else if (RelocInfo::IsEmbeddedObject(rmode)) {
     return EMBEDDED_OBJECT_SLOT;
   } else if (RelocInfo::IsDebugBreakSlot(rmode)) {
     return DEBUG_TARGET_SLOT;
   }
   UNREACHABLE();
-  return NUMBER_OF_SLOT_TYPES;
+  return CLEARED_SLOT;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_REMEMBERED_SET_H