[net/auth] Change diagnostics from strings to base::Value

Some of the diagnostics generated during a GSSAPI handshake are not
straightforward. The existing code constructs strings describing the
errors and logs them via LOG. In order to convert these to NetLogs, this
CL changes the logic to construct base::Values instead of freeform
strings.

R=mmenke@chromium.org

Bug: 884313
Change-Id: I084ab5eaf74df787f3f3c839c030e7c9fc5da3b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650628
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670676}