[Password] Migrate HttpAuthManagerTest to PasswordStoreInterface

PasswordStoreInterface is an interface that should be used by other
classes instead of interacting directly with PasswordStore.

Bug: 1218413
Change-Id: I9d4f6a7ab1447a84d40ccfe7289b1273ec4716b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3083742


Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#910216}