Removed special case for 416 and added comment for auth response cases.

The special handling for 416 is not needed and added comment for auth response
since in URLRequest::CancelAuth scenario HC::T::Read() can be invoked even
though the transaction is still entry->headers_transaction.

Bug: None
Change-Id: I9b031e8025b721a45f16957868d540b13c09fb09
Reviewed-on: https://chromium-review.googlesource.com/612400


Commit-Queue: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501356}