update_engine: replace std::vector<char> with chromeos::Blob

To make update engine consistent with the rest of platform2 code
replaced std::vector<char> as the container of binary data with
chromeos::Blob.

BUG=None
TEST=`FEATURES=test emerge-link update_engine`

Change-Id: I6385fd2257d15aa24bfa74ac35512c2a06c33012
Reviewed-on: https://chromium-review.googlesource.com/247793
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/certificate_checker_unittest.cc b/certificate_checker_unittest.cc
index a3d7465..5d4d8f0 100644
--- a/certificate_checker_unittest.cc
+++ b/certificate_checker_unittest.cc
@@ -61,7 +61,7 @@
   // Parameters of our mock certificate digest.
   int depth_;
   unsigned int length_;
-  unsigned char digest_[4];
+  uint8_t digest_[4];
   string digest_hex_;
   string diff_digest_hex_;
   string cert_key_prefix_;