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This is a reland of 8667827a

Original change's description:
> [Autofill Assistant] Adds action to use login credentials.
> 
> This CL extends SetFormFieldValueAction such that it can also fill a selector with a previously stored username or password. To this end, the action is refactored to allow for better code reuse and readability. Also, the CL adds unit tests (not just for the new functionality, but for the entire action).
> 
> 
> Bug: b/128833341
> Change-Id: I43e57bee5681047f1df954040f62265e0ab44f28
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728560
> Commit-Queue: Clemens Arbesser <arbesser@google.com>
> Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#689877}

Bug: b/128833341
Change-Id: I87eadf3a335dec9244d5adf94cd004326ed52597
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768422Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#690347}