Replace LoadPayloadMetadata() with PayloadMetadata class.

Removed duplicated payload header parsing logic.

Test: update_engine_unittests
Change-Id: I457c5cb86fa16e97b7a0c34d4039c46b86cd2957
diff --git a/payload_consumer/payload_metadata.h b/payload_consumer/payload_metadata.h
index fc1d128..8748f6f 100644
--- a/payload_consumer/payload_metadata.h
+++ b/payload_consumer/payload_metadata.h
@@ -55,6 +55,8 @@
   // the payload.
   MetadataParseResult ParsePayloadHeader(const brillo::Blob& payload,
                                          ErrorCode* error);
+  // Simpler version of the above, returns true on success.
+  bool ParsePayloadHeader(const brillo::Blob& payload);
 
   // Given the |payload|, verifies that the signed hash of its metadata matches
   // |metadata_signature| (if present) or the metadata signature in payload