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.cc b/delta_performer.cc
index 117e8f5..1a98102 100644
--- a/delta_performer.cc
+++ b/delta_performer.cc
@@ -33,12 +33,13 @@
 
 namespace chromeos_update_engine {
 
+const char DeltaPerformer::kUpdatePayloadPublicKeyPath[] =
+    "/usr/share/update_engine/update-payload-key.pub.pem";
+
 namespace {
 
 const int kDeltaVersionLength = 8;
 const int kDeltaProtobufLengthLength = 8;
-const char kUpdatePayloadPublicKeyPath[] =
-    "/usr/share/update_engine/update-payload-key.pub.pem";
 const int kUpdateStateOperationInvalid = -1;
 const int kMaxResumedUpdateFailures = 10;