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/bzip_extent_writer.h b/bzip_extent_writer.h
index 0187c0b..6a6133a 100644
--- a/bzip_extent_writer.h
+++ b/bzip_extent_writer.h
@@ -5,8 +5,11 @@
 #ifndef UPDATE_ENGINE_BZIP_EXTENT_WRITER_H_
 #define UPDATE_ENGINE_BZIP_EXTENT_WRITER_H_
 
-#include <vector>
 #include <bzlib.h>
+#include <vector>
+
+#include <chromeos/secure_blob.h>
+
 #include "update_engine/extent_writer.h"
 #include "update_engine/utils.h"
 
@@ -32,7 +35,7 @@
  private:
   ExtentWriter* const next_;  // The underlying ExtentWriter.
   bz_stream stream_;  // the libbz2 stream
-  std::vector<char> input_buffer_;
+  chromeos::Blob input_buffer_;
 };
 
 }  // namespace chromeos_update_engine