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_consumer/download_action.h b/payload_consumer/download_action.h
index 0bd0d88..48d6292 100644
--- a/payload_consumer/download_action.h
+++ b/payload_consumer/download_action.h
@@ -131,6 +131,9 @@
   // called or if CloseP2PSharingFd() has been called.
   void WriteToP2PFile(const void* data, size_t length, off_t file_offset);
 
+  // Start downloading the current payload using delta_performer.
+  void StartDownloading();
+
   // The InstallPlan passed in
   InstallPlan install_plan_;