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* Prepares the safety check UI to receive a pluralized string to show
  in the 'Change password[s]' button, depending on if one or multiple
  passwords are compromised.
* Adds the C++ side string construction w/ tests.

Bug: 1015841
Change-Id: I4cf1b2197536f2cd7fbc606f8f167976cfc74f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107549Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751338}