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Updates the credential store with changes from the password store.
Renames SyncAllCredentials to RequestSyncAllCredentials to make clearer
the async nature of it.
Creates entry functions for adding, updating, and removing passwords
which will be used to also keep up to date the OS metadata in a follow
up CL.
Injects the credential store for easier testing.

Bug: 1066803
Change-Id: Id79c3b276517c7bb223b3988db4d1a8c25e00025
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152814
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760483}