Apply multiple payload in DownloadAction.

DownloadAction will now call DeltaPerformer once for each payload.

Bug: 36252799
Test: update two payload works!
Test: update_engine_unittest

Change-Id: Ibd4f3a4af9f701852f2c70b9384ca9d1fedacbeb
(cherry picked from commit 7bd9f0b95403664229b701a13dca03c31a46fb49)
diff --git a/payload_state.h b/payload_state.h
index 14f0f50..56e32fd 100644
--- a/payload_state.h
+++ b/payload_state.h
@@ -153,6 +153,14 @@
     return attempt_error_code_;
   }
 
+  bool NextPayload() override {
+    if (payload_index_ + 1 >= candidate_urls_.size())
+      return false;
+    payload_index_++;
+    url_index_ = 0;
+    return true;
+  }
+
  private:
   enum class AttemptType {
     kUpdate,