Signature size now uses size_t

Signature size shouldn't have negative values. Make its usage more
consistent.

Test: mma
Change-Id: Ie6cbf49694fcbe0348d6338373423d77d3edfdd4
diff --git a/payload_generator/payload_signer.h b/payload_generator/payload_signer.h
index 7854e12..76e583b 100644
--- a/payload_generator/payload_signer.h
+++ b/payload_generator/payload_signer.h
@@ -91,7 +91,7 @@
   //
   // The changes to payload are not preserved or written to disk.
   static bool HashPayloadForSigning(const std::string& payload_path,
-                                    const std::vector<int>& signature_sizes,
+                                    const std::vector<size_t>& signature_sizes,
                                     brillo::Blob* out_payload_hash_data,
                                     brillo::Blob* out_metadata_hash);