Don't key client auth on HttpNetworkSession.

It's an IO-thread object being maintained (fortunately just as a pointer) on
the UI thread. It's used to tie together SSLClientAuthObserver instances, but
the BrowserContext is a more suitable UI thread key.

This relands part of https://codereview.chromium.org/596873002

BUG=439134

Review URL: https://codereview.chromium.org/780383002

Cr-Commit-Position: refs/heads/master@{#307274}
20 files changed