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/mock_certificate_checker.h b/mock_certificate_checker.h
index 5731062..8c26b42 100644
--- a/mock_certificate_checker.h
+++ b/mock_certificate_checker.h
@@ -18,7 +18,7 @@
                      bool(X509_STORE_CTX* x509_ctx,
                           int* out_depth,
                           unsigned int* out_digest_length,
-                          unsigned char* out_digest));
+                          uint8_t* out_digest));
 };
 
 }  // namespace chromeos_update_engine