[iOS] Do not drop cookie tasks when cookie loading is interrupted.

[iOS] Do not drop cookie tasks when cookie loading is interrupted.

This CL adds support for the case where cookie loading is interrupted
with pending tasks.
The old behavior was to DCHECK and early return without processing the
tasks until the next reload of the cookies.
This CL implements the right behavior: run the tasks against the
CookieMonster which becomes the source of truth when cookie loading is
interrupted.

The CL also adds two unittests that would have been failing before.

BUG=469843

Review URL: https://codereview.chromium.org/1187283002

Cr-Commit-Position: refs/heads/master@{#335506}