Disable captive portal URL randomization by default

Captive portal URL randomization causes problems in some environments
so disable it by default.

Bug: 776409
Change-Id: I0040091708096f51a5877e94bc036ebfadbbcca5
Reviewed-on: https://chromium-review.googlesource.com/841426
Commit-Queue: Qiang(Joe) Xu <[email protected]>
Reviewed-by: Albert Bodenhamer <[email protected]>
Reviewed-by: Qiang(Joe) Xu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525914}
7 files changed