delta_generator: Include metadata signature in major version 2.

--metadata_signature_file flag does what it should now.
Note that you should only pass this flag if the payload you are signing is
version 2.

Bug: 23981164
TEST=unit test added.

Change-Id: I613cd6a5fef188eca37c46c3f8a0a41d1c22f2fd
diff --git a/delta_performer_integration_test.cc b/delta_performer_integration_test.cc
index 517891b..c483488 100644
--- a/delta_performer_integration_test.cc
+++ b/delta_performer_integration_test.cc
@@ -174,6 +174,7 @@
   return PayloadSigner::AddSignatureToPayload(
       payload_path,
       signatures,
+      {},
       payload_path,
       out_metadata_size);
 }
@@ -194,6 +195,7 @@
   ASSERT_TRUE(PayloadSigner::AddSignatureToPayload(
       payload_path,
       vector<brillo::Blob>(1, signature),
+      {},
       payload_path,
       out_metadata_size));
   EXPECT_TRUE(PayloadSigner::VerifySignedPayload(