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This CL defers adding the "Use other [password|username]" button
until the newly introduced helper confirmed that there is at
least one credential to show.

To do that, the helper fetches the passwords when it's created.
If it finishes before suggestions are to be refreshed, the
stored boolean is used, otherwise, the callback is queued and
will trigger an update.

Smaller clean-ups:
* New test fixture for filling (mostly moved to save setup code
  and to prevent ScopedFeatureList issues)
* Moved accessory sheet state (focused field) into helper since
  frame state shouldn't be used by the accessory controller
  (if possible but required by all pwds sheet)

Bug: 1104132
Change-Id: I4fa5757744dda5c2091fec24daf0654f92b2be46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489914
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825468}