Adding an about:flag for URL Loader HTTPS server previews

This adds an about://flag entry for the new way of loading an HTTPS
server preview using the network service.

Bug: 921736
Change-Id: Ib9c419abdcc09d1700a98854d7e3863f7b4edbc0
Reviewed-on: https://chromium-review.googlesource.com/c/1490442
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636231}
5 files changed