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/delta_performer.cc b/delta_performer.cc
index de877b8..6094c5a 100644
--- a/delta_performer.cc
+++ b/delta_performer.cc
@@ -68,6 +68,8 @@
 const unsigned DeltaPerformer::kProgressDownloadWeight = 50;
 const unsigned DeltaPerformer::kProgressOperationsWeight = 50;
 
+const uint64_t kChromeOSMajorPayloadVersion = 1;
+const uint64_t kBrilloMajorPayloadVersion = 2;
 const uint32_t kInPlaceMinorPayloadVersion = 1;
 const uint32_t kSourceMinorPayloadVersion = 2;