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/install_plan.h b/install_plan.h
index c592889..782f2ab 100644
--- a/install_plan.h
+++ b/install_plan.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include <base/macros.h>
+#include <chromeos/secure_blob.h>
 
 #include "update_engine/action.h"
 
@@ -62,8 +63,8 @@
   // applied partition sizes and hashes against the expected values.
   uint64_t kernel_size;
   uint64_t rootfs_size;
-  std::vector<char> kernel_hash;
-  std::vector<char> rootfs_hash;
+  chromeos::Blob kernel_hash;
+  chromeos::Blob rootfs_hash;
 
   // True if payload hash checks are mandatory based on the system state and
   // the Omaha response.