[Passwords] Make password store non-abstract

This CL doesn't intend to change any behavior.

In order to swap out the platform-specific parts of the PasswordStore,
the store can be created if the correct backend is passed in.

Right now, the only backend still is derived from the store which means
this change is most notable in tests where the store doesn't need to
write to the disc anymore but can use a mock to echo some passwords.
A couple of pure virtual methods from the old implementation pattern are
stubbed to make the standalone creation possible.

In a follow-up, an Android-specific backend makes use of the newly added
constructor.
In another follow-up, more tests will move from password_store to
password_store_impl to test the backend functionality independent from
the platform.


Bug: 1217071
Change-Id: I79d2341c87c77fc518c22bb3ed72d87ad6a54055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3010374
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#902392}