Improve PaymentRequestMediatorTest unit tests

Instead of using IdentityTestEnvironmentChromeBrowserState adaptor,
convert the tests to work with a real AuthenticationService (backed
by a fake ChromeIdentityService).

See design doc:
https://docs.google.com/document/d/11cqV7S3WCaeOaP63WQWvzNdvSFflgzwZs8GCgdwQ7Bc/preview

Bug: 983171
Change-Id: Iae78a8fbbf0b2902a037973e93c6c897072dcdcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700211
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677696}
2 files changed