[HybridInAutofill] Restrict entry point to webauthn fields

Only show the entry point for Hybrid passkey flows on fields that have
the to autocomplete="webauthn" attribute. This ensures it's not shown on
fields where it's less useful.

Screenshot of the entry point: http://screen/7RacahewbUDhBVc

Feature: AutofillReintroduceHybridPasskeyDropdownItem
Bug: 399124614
Fixed: 440918921
Change-Id: Ia2767ae4520fbd8df7b44682f14a0c3b21817ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6874914
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Friedrich Hauser <friedrichh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506394}