AU: DeltaPerformer performs the download size/hash check now.

DownloadAction still calculates a hash in this case, however,
we can remove it when we remove old-style download support.

BUG=7393
TEST=unit tests, gmerge on device and updated with signed load,
with different download size/hash.

Change-Id: I5af9a09f87264159fc55070735463ad920fd7373

Review URL: http://codereview.chromium.org/3547019
diff --git a/prefs_interface.h b/prefs_interface.h
index a9b20a5..66fe8cb 100644
--- a/prefs_interface.h
+++ b/prefs_interface.h
@@ -16,6 +16,7 @@
 extern const char kPrefsUpdateCheckResponseHash[];
 extern const char kPrefsUpdateStateNextDataOffset[];
 extern const char kPrefsUpdateStateNextOperation[];
+extern const char kPrefsUpdateStateSHA256Context[];
 extern const char kPrefsUpdateStateSignedSHA256Context[];
 
 // The prefs interface allows access to a persistent preferences