Fix some more shorten-64-to-32 warnings:

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index f63561806c5d5b01bf4abb635a7d053c85db5caa..b1071d61da35dab764cb1388b7546508371e659f 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -121,16 +121,12 @@ static const bool kAsInlinedObject = false;
 static const intptr_t kInvalidPatchIndex = -1;
 
 
-class SerializedHeaderTag : public BitField<enum SerializedHeaderType,
-                                            0,
-                                            kHeaderTagBits> {
-};
+class SerializedHeaderTag :
+    public BitField<intptr_t, enum SerializedHeaderType, 0, kHeaderTagBits> {};
 
 
-class SerializedHeaderData : public BitField<intptr_t,
-                                             kHeaderTagBits,
-                                             kObjectIdBits> {
-};
+class SerializedHeaderData :
+    public BitField<intptr_t, intptr_t, kHeaderTagBits, kObjectIdBits> {};
 
 
 enum DeserializeState {