AU: Support signed payload verification through the delta generator.

The following command will verify a signed payload:

./delta_generator -in_file <payload> -public_key <key.pem>

Add a unit test for signature verification.

BUG=chromium-os:10936
TEST=unit tests

Change-Id: I3bb464608c22229141f9819b27503b6de778c495

Review URL: http://codereview.chromium.org/6271003
diff --git a/delta_performer_unittest.cc b/delta_performer_unittest.cc
index 0d71a6e..bd13512 100755
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -238,6 +238,8 @@
     ASSERT_TRUE(PayloadSigner::AddSignatureToPayload(delta_path,
                                                      signature,
                                                      delta_path));
+    EXPECT_TRUE(PayloadSigner::VerifySignedPayload(delta_path,
+                                                   kUnittestPublicKeyPath));
   }
 
   // Read delta into memory.