Make cookie_store_unittest.h use only one CookieStore per test.

This is a preliminary CL before landing a cross-thread CookieStore class
that wraps a global cookie store for Android Webview, and will allow the
standard tests to be run with the new store.

Tests for the new store will still need to delete all cookies before
each test, but this allows for a saner test API.

BUG=579653, 579260

Review URL: https://codereview.chromium.org/1691263003

Cr-Commit-Position: refs/heads/master@{#375362}