[Autofill] Hide profile dedupe behind a finch flag

Bug: 1168067
Change-Id: I4f19e75d50a11e11ec19d2f5dac1e3f4f0558a73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2673546


Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Vidhan Jain <vidhanj@google.com>
Cr-Commit-Position: refs/heads/master@{#851128}