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/libcurl_http_fetcher.cc b/libcurl_http_fetcher.cc
index a7f6490..e32d925 100644
--- a/libcurl_http_fetcher.cc
+++ b/libcurl_http_fetcher.cc
@@ -92,7 +92,7 @@
   if (post_data_set_) {
     CHECK_EQ(curl_easy_setopt(curl_handle_, CURLOPT_POST, 1), CURLE_OK);
     CHECK_EQ(curl_easy_setopt(curl_handle_, CURLOPT_POSTFIELDS,
-                              &post_data_[0]),
+                              post_data_.data()),
              CURLE_OK);
     CHECK_EQ(curl_easy_setopt(curl_handle_, CURLOPT_POSTFIELDSIZE,
                               post_data_.size()),
@@ -383,7 +383,7 @@
   bytes_downloaded_ += payload_size;
   in_write_callback_ = true;
   if (delegate_)
-    delegate_->ReceivedBytes(this, reinterpret_cast<char*>(ptr), payload_size);
+    delegate_->ReceivedBytes(this, ptr, payload_size);
   in_write_callback_ = false;
   return payload_size;
 }