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_unittest.cc b/delta_performer_unittest.cc
index 0451d9a..f0f399d 100644
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -543,6 +543,7 @@
     payload_config.is_delta = !full_rootfs;
     payload_config.hard_chunk_size = chunk_size;
     payload_config.rootfs_partition_size = kRootFSPartitionSize;
+    payload_config.major_version = kChromeOSMajorPayloadVersion;
     payload_config.minor_version = minor_version;
     if (!full_rootfs) {
       payload_config.source.rootfs.path = state->a_img;