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/omaha_request_action.h b/omaha_request_action.h
index d086a24..b9f70f0 100644
--- a/omaha_request_action.h
+++ b/omaha_request_action.h
@@ -13,6 +13,7 @@
 #include <string>
 #include <vector>
 
+#include <chromeos/secure_blob.h>
 #include <curl/curl.h>
 
 #include "update_engine/action.h"
@@ -139,7 +140,7 @@
 
   // Delegate methods (see http_fetcher.h)
   void ReceivedBytes(HttpFetcher *fetcher,
-                     const char* bytes, int length) override;
+                     const void* bytes, size_t length) override;
 
   void TransferComplete(HttpFetcher *fetcher, bool successful) override;
 
@@ -285,7 +286,7 @@
   bool ping_only_;
 
   // Stores the response from the omaha server
-  std::vector<char> response_buffer_;
+  chromeos::Blob response_buffer_;
 
   // Initialized by InitPingDays to values that may be sent to Omaha
   // as part of a ping message. Note that only positive values and -1