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 <[email protected]>
Reviewed-by: Alex Deymo <[email protected]>
Tested-by: Alex Vakulenko <[email protected]>
Commit-Queue: Alex Vakulenko <[email protected]>
diff --git a/postinstall_runner_action_unittest.cc b/postinstall_runner_action_unittest.cc
index 15169e4..fc61f4e 100644
--- a/postinstall_runner_action_unittest.cc
+++ b/postinstall_runner_action_unittest.cc
@@ -116,8 +116,8 @@
   string orig_cwd;
   {
     vector<char> buf(1000);
-    ASSERT_EQ(&buf[0], getcwd(&buf[0], buf.size()));
-    orig_cwd = string(&buf[0], strlen(&buf[0]));
+    ASSERT_EQ(buf.data(), getcwd(buf.data(), buf.size()));
+    orig_cwd = string(buf.data(), strlen(buf.data()));
   }
 
   // Create a unique named working directory and chdir into it.