[serializer] small fixes for blink snapshot.

src/snapshot/deserializer.cc

 // 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.
 
 #include "src/snapshot/deserializer.h"
 
 #include "src/bootstrapper.h"
 #include "src/external-reference-table.h"
 #include "src/heap/heap.h"
 #include "src/isolate.h"
@@ skipping 510 matching lines @@
         emit_write_barrier = isolate->heap()->InNewSpace(new_object);          \
         hot_objects_.Add(HeapObject::cast(new_object));                        \
       } else if (where == kPartialSnapshotCache) {                             \
         int cache_index = source_.GetInt();                                    \
         new_object = isolate->partial_snapshot_cache()->at(cache_index);       \
         emit_write_barrier = isolate->heap()->InNewSpace(new_object);          \
       } else if (where == kExternalReference) {                                \
         int skip = source_.GetInt();                                           \
         current = reinterpret_cast<Object**>(                                  \
             reinterpret_cast<Address>(current) + skip);                        \
-        int reference_id = source_.GetInt();                                   \
+        uint32_t reference_id = static_cast<uint32_t>(source_.GetInt());       \
         Address address = external_reference_table_->address(reference_id);    \
         new_object = reinterpret_cast<Object*>(address);                       \
       } else if (where == kAttachedReference) {                                \
         int index = source_.GetInt();                                          \
         new_object = *attached_objects_[index];                                \
         emit_write_barrier = isolate->heap()->InNewSpace(new_object);          \
       } else {                                                                 \
         DCHECK(where == kBuiltin);                                             \
         DCHECK(deserializing_user_code());                                     \
         int builtin_id = source_.GetInt();                                     \
@@ skipping 309 matching lines @@
 
       default:
         CHECK(false);
     }
   }
   CHECK_EQ(limit, current);
   return true;
 }
 }  // namespace internal
 }  // namespace v8