AU: Verify delta payload signature and signed hash.

The signature and hash are verified only if the public key file exists.
This means that currently this feature is desabled until we install a public
key.

BUG=5663
TEST=unit tests, applied a signed delta update on the server

Change-Id: I5be72f7fde88400587f8aae0c7d5745c79fc4428

Review URL: http://codereview.chromium.org/3592008
diff --git a/download_action.cc b/download_action.cc
index 6b27722..608eeb9 100644
--- a/download_action.cc
+++ b/download_action.cc
@@ -150,6 +150,11 @@
                  << " failed. Expected size " << install_plan_.size
                  << " but got size " << bytes_received_;
       code = kActionCodeDownloadSizeMismatchError;
+    } else if (!install_plan_.is_full_update &&
+               !delta_performer_->VerifyPayload("")) {
+      LOG(ERROR) << "Download of " << install_plan_.download_url
+                 << " failed due to payload verification error.";
+      code = kActionCodeDownloadPayloadVerificationError;
     }
   }