AU: Switch from SHA-1 to SHA-256 hash.

For hashing of images, expect a new "sha256" attribute in Omaha's updatecheck
response.

BUG=6580
TEST=unit tests, gmerged on device and updated with upgraded dev server

Change-Id: I122e21cd7edc74695cb81b2eee5ce70f152f5270

Review URL: http://codereview.chromium.org/3419018
diff --git a/omaha_hash_calculator.h b/omaha_hash_calculator.h
index 5a666b6..208fd01 100644
--- a/omaha_hash_calculator.h
+++ b/omaha_hash_calculator.h
@@ -57,7 +57,7 @@
   bool valid_;
 
   // The hash state used by OpenSSL
-  SHA_CTX ctx_;
+  SHA256_CTX ctx_;
   DISALLOW_COPY_AND_ASSIGN(OmahaHashCalculator);
 };