Add major_version flag in delta_generator.

Only affects major version number in the payload for now, actual implementation
of major version 2 will follow.

TEST=Call delta_generator manually with --major_version=2 then cros payload
show.

Bug: 23694580
Change-Id: I5baca5a868ec00dc2f6ea63d89e295d43283fd32
diff --git a/payload_generator/payload_file.h b/payload_generator/payload_file.h
index 51c699c..6e0776c 100644
--- a/payload_generator/payload_file.h
+++ b/payload_generator/payload_file.h
@@ -81,6 +81,9 @@
   // payload file.
   static const std::vector<PartitionName> partition_disk_order_;
 
+  // The major_version of the requested payload.
+  uint64_t major_version_;
+
   DeltaArchiveManifest manifest_;
 
   std::map<PartitionName, std::vector<AnnotatedOperation>> aops_map_;