[ios] first step into adding app level url loading service

Will allow factoring some url loading code out of main controller.
App level service will take care of ‘global’ action, like incognito
swap, modals closing, etc.

The (singleton) app level loading service is piped down to the
browser state level url loading services.

Bug: 907527
Change-Id: Icc7c93204a67aa6d958be7f3d352e37766e8909c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496876
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639904}
16 files changed