Serializer: canonicalize cleared weak cells.

src/snapshot/serialize.h

Index: src/snapshot/serialize.h
diff --git a/src/snapshot/serialize.h b/src/snapshot/serialize.h
index 36514e13d391927aa247a4714ff519194effacad..f6a88f05436fb753435061d1a538754106eb5e48 100644
--- a/src/snapshot/serialize.h
+++ b/src/snapshot/serialize.h
@@ -373,6 +373,8 @@ class SerializerDeserializer: public ObjectVisitor {
   static const int kNop = 0x3d;
   // Move to next reserved chunk.
   static const int kNextChunk = 0x3e;
+  // Canonical cleared weak cell.
+  static const int kClearedWeakCell = 0x3f;
   // A tag emitted at strategic points in the snapshot to delineate sections.
   // If the deserializer does not find these at the expected moments then it
   // is an indication that the snapshot and the VM do not fit together.