[serializer] prepare attached references for general use.

src/snapshot/serializer-common.h

Index: src/snapshot/serializer-common.h
diff --git a/src/snapshot/serializer-common.h b/src/snapshot/serializer-common.h
index 645a9af3bf647ded9672abfe47cb042b709601e1..1ce5cedfae89e27f2b67e78521efa92d1b7ecada 100644
--- a/src/snapshot/serializer-common.h
+++ b/src/snapshot/serializer-common.h
@@ -40,11 +40,13 @@ class HotObjectsList {
   }
 
   void Add(HeapObject* object) {
+    DCHECK(!AllowHeapAllocation::IsAllowed());
     circular_queue_[index_] = object;
     index_ = (index_ + 1) & kSizeMask;
   }
 
   HeapObject* Get(int index) {
+    DCHECK(!AllowHeapAllocation::IsAllowed());
     DCHECK_NOT_NULL(circular_queue_[index]);
     return circular_queue_[index];
   }
@@ -52,6 +54,7 @@ class HotObjectsList {
   static const int kNotFound = -1;
 
   int Find(HeapObject* object) {
+    DCHECK(!AllowHeapAllocation::IsAllowed());
     for (int i = 0; i < kSize; i++) {
       if (circular_queue_[i] == object) return i;
     }
@@ -211,12 +214,6 @@ class SerializerDeserializer : public ObjectVisitor {
   // Sentinel after a new object to indicate that double alignment is needed.
   static const int kDoubleAlignmentSentinel = 0;
 
-  // Used as index for the attached reference representing the source object.
-  static const int kSourceObjectReference = 0;
-
-  // Used as index for the attached reference representing the global proxy.
-  static const int kGlobalProxyReference = 0;
-
   // ---------- member variable ----------
   HotObjectsList hot_objects_;
 };