[Passwords] Introduce FormFetcherImpl::CreateFormFetcherImpl() factory

This is a factory method that would actually instantiate a
FormFetcherImpl or a MultiStoreFormFetcher.

Change-Id: Ib498a1996b6ce98593fcef288a2f7c330e31acf1
Bug: 1002000
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803143
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696416}