Modify autofill update credential pixel to include backfilled parameter

[CDRussell]

Task/Issue URL: https://app.asana.com/0/1202926619870900/1208976665268815/f

Description

Steps to test this PR

Logcat filter: message~:"Partial save" message~:"Pixel sent: m_autofill_logins_update_password_inline" message~:"Determined that username" message~:"Determined that username”

Simulating a multi-step login form

• Ensure no passwords currently saved (or none for autofill.me at least)
• Visit https://autofill.me/form/login-simple
• Enter username test
• Leave password blank
• Hit Login button
• Now clear the username
• Enter password (>=4 characters)
• Hit Login button
• Save password when prompted, then go view it in the Password Management view (e.g., tap on View in snackbar)
• Verify it has username=test and password matches what you provided (i.e., the username was backfilled because the partial form submission’s username was later applied to the final form submission which didn’t have the username)
• [ ]

[CDRussell]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Modify autofill update credential pixel to include backfilled parameter #5392 👈 (View in Graphite)
• Autofill: Increase ratio of complete credential saves #5386
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