If a public key is present, disallow old style full payloads.

This is necessary to ensure that if a public key is present the payload is
signed and the signature passes verification.

BUG=chromium-os:11760
TEST=unit tests, tested on device

Change-Id: I6af61ead0e918c0b971dbcfeabcab3be03e6eb97

Review URL: http://codereview.chromium.org/6574009
diff --git a/delta_performer.h b/delta_performer.h
index c693333..7cc1d9b 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -31,6 +31,8 @@
     kMetadataParseInsufficientData,
   };
 
+  static const char kUpdatePayloadPublicKeyPath[];
+
   DeltaPerformer(PrefsInterface* prefs)
       : prefs_(prefs),
         fd_(-1),