GitLab

 - Add a method for reading a payload as another NtlmBufferReader
   with a subset range.

BUG=chromium:22532

Change-Id: I6f2f16f88b864fefbc46d93f2eab5aa780ec7506
Reviewed-on: https://chromium-review.googlesource.com/656137
Commit-Queue: Zentaro Kavanagh <zentaro@google.com>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505184}