GitLab

Fix crashes caused by |url_checker_for_testing_| not being initialized
properly.

Additionally, construct UrlValidityChecker with NoDestructor instead of
a global LazyInstance. We prefer to leak singletons to avoid
unnecessary work at shutdown. Also simplify UrlValidityChecker by
passing TickClock, which provides cleaner timeout and duration testing.

Bug: 894742
Change-Id: I468f76e07dc6f551a8515d8416d0c67f5f2035a1
Reviewed-on: https://chromium-review.googlesource.com/c/1277959
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599377}