Add flag for HTTP Auth committed interstitials.

This CL adds a flag and feature for enabling HTTP auth via committed
interstitials. Nothing uses it yet.

Change-Id: Ia7d2e3b0c66b22cfd5fa56c6ec65f2289a2665d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589013
Commit-Queue: Joe DeBlasio <[email protected]>
Auto-Submit: Joe DeBlasio <[email protected]>
Reviewed-by: Emily Stark <[email protected]>
Cr-Commit-Position: refs/heads/master@{#655106}
7 files changed