Support IsDeltaPayload per <package>.

Mixed full and delta payloads are now supported for multi-payload.

Bug: 36252799
Test: update_engine_unittests
Change-Id: Ic519f8399579c714a65ba9f9f948a389fcc249ed
(cherry picked from commit 4928112f8635f7a5b583171640aee2bda690ba34)
diff --git a/omaha_response.h b/omaha_response.h
index 395891f..c702068 100644
--- a/omaha_response.h
+++ b/omaha_response.h
@@ -46,6 +46,9 @@
     uint64_t metadata_size = 0;
     std::string metadata_signature;
     std::string hash;
+    // True if the payload described in this response is a delta payload.
+    // False if it's a full payload.
+    bool is_delta = false;
   };
   std::vector<Package> packages;
 
@@ -58,10 +61,6 @@
   uint32_t max_failure_count_per_url = 0;
   bool prompt = false;
 
-  // True if the payload described in this response is a delta payload.
-  // False if it's a full payload.
-  bool is_delta_payload = false;
-
   // True if the Omaha rule instructs us to disable the back-off logic
   // on the client altogether. False otherwise.
   bool disable_payload_backoff = false;