AU: Try delta updates first, then full updates

src/platform/update_engine/update_metadata.proto

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Update file format: A delta update file contains all the deltas needed
 // to update a system from one specific version to another specific
 // version. The update format is represented by this struct pseudocode:
 // struct delta_update_file {
 //   char magic[4] = "CrAU";
 //   uint64 bom_offset;  // Offset of protobuf DeltaArchiveManifest
@@ skipping 84 matching lines @@
 
     // File Data, not for directories
     enum DataFormat {
       FULL = 0;  // The data is the complete file
       FULL_GZ = 1;  // The data is the complete file gzipped
       BSDIFF = 2;  // The data is a bsdiff binary diff
       COURGETTE = 3;  // The data is a courgette binary diff
     }
     // If present, there is data associated with this File object and
     // data_offset and data_size must be set.
+    // If a file object doesn't have this set, it means the data is
+    // unchanged from the old version of the file.
     optional DataFormat data_format = 4;
     // The offset into the delta file where the data (if any) is stored
     optional uint32 data_offset = 5;
     // The length of the data in the delta file
     optional uint32 data_length = 6;
     
     // When a file is a hard link, hardlink_path exists and
     // is the path within the DeltaArchiveManifest to the "original" file.
     // When iterating through a DeltaArchiveManifest,  you will be guaranteed
     // to hit a hardlink only after you've hit the path to the first file.
@@ skipping 10 matching lines @@
     }
     repeated Child children = 9;
   }
   repeated File files = 1;
 }
 
 message LinuxDevice {
   required int32 major = 1;
   required int32 minor = 2;
 }