AU: No need to calculate SHA-256 on the payload twice for delta updates.

download_action.cc

Index: download_action.cc
diff --git a/download_action.cc b/download_action.cc
index bb5d18938c7d302a0c062455ab40a9b0fd3d8102..c17d7e26f7af1a17c5b440477e7ae79788e0ff67 100644
--- a/download_action.cc
+++ b/download_action.cc
@@ -123,7 +123,10 @@ void DownloadAction::ReceivedBytes(HttpFetcher *fetcher,
     processor_->ActionComplete(this, kActionCodeDownloadWriteError);
     return;
   }
-  omaha_hash_calculator_.Update(bytes, length);
+  // DeltaPerformer checks the hashes for delta updates.
+  if (install_plan_.is_full_update) {
+    omaha_hash_calculator_.Update(bytes, length);
+  }
 }
 
 namespace {