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.cc b/prefs.cc
index 2de7eea..733443c 100644
--- a/prefs.cc
+++ b/prefs.cc
@@ -22,6 +22,7 @@
 const char kPrefsUpdateCheckResponseHash[] = "update-check-response-hash";
 const char kPrefsUpdateStateNextDataOffset[] = "update-state-next-data-offset";
 const char kPrefsUpdateStateNextOperation[] = "update-state-next-operation";
+const char kPrefsUpdateStateSHA256Context[] = "update-state-sha-256-context";
 const char kPrefsUpdateStateSignedSHA256Context[] =
     "update-state-signed-sha-256-context";