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.cc b/payload_consumer/payload_metadata.cc
index f700228..02ec8b1 100644
--- a/payload_consumer/payload_metadata.cc
+++ b/payload_consumer/payload_metadata.cc
@@ -126,6 +126,11 @@
   return MetadataParseResult::kSuccess;
 }
 
+bool PayloadMetadata::ParsePayloadHeader(const brillo::Blob& payload) {
+  ErrorCode error;
+  return ParsePayloadHeader(payload, &error) == MetadataParseResult::kSuccess;
+}
+
 bool PayloadMetadata::GetManifest(const brillo::Blob& payload,
                                   DeltaArchiveManifest* out_manifest) const {
   uint64_t manifest_offset;