AU: detect failure when using public key verification

delta_performer.h

Index: delta_performer.h
diff --git a/delta_performer.h b/delta_performer.h
index 7cc1d9bf7378804fda2fb8a3b12dc30d75b3174e..a80e180f8c798e234b08512c04c3fbc36bc1c3aa 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -67,9 +67,12 @@ class DeltaPerformer : public FileWriter {
   // is unavailable; it returns false if the public key is available but the
   // delta payload doesn't include a signature. If |public_key_path| is an empty
   // string, uses the default public key path.
+  // If the signature check fails, *signature_failed will be set to true (if
+  // it's non-NULL); this will not cause the method to fail.
   bool VerifyPayload(const std::string& public_key_path,
                      const std::string& update_check_response_hash,
-                     const uint64_t update_check_response_size);
+                     const uint64_t update_check_response_size,
+                     bool* signature_failed);
 
   // Reads from the update manifest the expected sizes and hashes of the target
   // kernel and rootfs partitions. These values can be used for applied update