[Sync::USS] PasswordStoreX supports USS when there is no backend

PasswordStoreX has 3 methods that are specific to Sync USS.
Those method are currently marked as NOIMPLEMENTED because USS doesn't
support PasswordStoreX and the plan is to migrate away from
PasswordStoreX before launching USS for Passwords.

This CL changes this behavior such that in case there is no backend,
PasswordStoreX will delegate those methods to PasswordStoreDefault which
fully supports USS. This behavior is similar to other methods in

Same as of before this CL, it's the responsibility of Sync code base to
make sure those methods are called only when it makes sense.

This change will enable to run the sync integration tests on Linux
because they don't instantiate a backend.

Bug: 902349
Change-Id: Ie1976e3521300829d31094324832a33382f19306
Reviewed-on: https://chromium-review.googlesource.com/c/1442254
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627386}
1 file changed