Use string for Signatures protobuf.

To avoid conversion between brillo::Blob and string.
Also use SerializeToString() instead of AppendToString().

Test: update_engine_unittests
Change-Id: I0ad0fb5f45223f9f51f940de727660a9e62052bb
diff --git a/payload_generator/payload_signer.h b/payload_generator/payload_signer.h
index b2d6606..7854e12 100644
--- a/payload_generator/payload_signer.h
+++ b/payload_generator/payload_signer.h
@@ -54,17 +54,17 @@
                        brillo::Blob* out_signature);
 
   // Sign |hash_data| blob with all private keys in |private_key_paths|, then
-  // convert the signatures to protobuf blob.
+  // convert the signatures to serialized protobuf.
   static bool SignHashWithKeys(
       const brillo::Blob& hash_data,
       const std::vector<std::string>& private_key_paths,
-      brillo::Blob* out_signature_blob);
+      std::string* out_serialized_signature);
 
   // Given an unsigned payload in |unsigned_payload_path|, private keys in
   // |private_key_path|, metadata size in |metadata_size|, metadata signature
   // size in |metadata_signature_size| and signatures offset in
   // |signatures_offset|, calculates the payload signature blob into
-  // |out_signature_blob|. Note that the payload must already have an
+  // |out_serialized_signature|. Note that the payload must already have an
   // updated manifest that includes the dummy signature op and correct metadata
   // signature size in header. Returns true on success, false otherwise.
   static bool SignPayload(const std::string& unsigned_payload_path,
@@ -72,9 +72,9 @@
                           const uint64_t metadata_size,
                           const uint32_t metadata_signature_size,
                           const uint64_t signatures_offset,
-                          brillo::Blob* out_signature_blob);
+                          std::string* out_serialized_signature);
 
-  // Returns the length of out_signature_blob that will result in a call
+  // Returns the length of out_serialized_signature that will result in a call
   // to SignPayload with the given private keys. Returns true on success.
   static bool SignatureBlobLength(
       const std::vector<std::string>& private_key_paths, uint64_t* out_length);