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/fake_file_writer.h b/fake_file_writer.h
index 6ba550d..751cbcf 100644
--- a/fake_file_writer.h
+++ b/fake_file_writer.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include <base/macros.h>
+#include <chromeos/secure_blob.h>
 
 #include "update_engine/file_writer.h"
 
@@ -43,13 +44,13 @@
     return 0;
   }
 
-  const std::vector<char>& bytes() {
+  const chromeos::Blob& bytes() {
     return bytes_;
   }
 
  private:
   // The internal store of all bytes that have been written
-  std::vector<char> bytes_;
+  chromeos::Blob bytes_;
 
   // These are just to ensure FileWriter methods are called properly.
   bool was_opened_;