[mullhet] Display the 'Suggest password' option only on password fields

Unlike the other password-related context menu entries, the
'Suggest password' entry can only be displayed on password fields. If
displayed on a non-password field, the entry is a no-op (passwords
cannot be generated on non-password fields).

The logic introduced in this CL is copied from where it is decided
(in non-mullhet scenarios) whether to display the 'Suggest password'
entry or not:

https://source.chromium.org/chromium/chromium/src/+/main:components/renderer_context_menu/context_menu_content_type.cc;l=158;drc=eca0d3645e88876fefaf2ed1f1e6164fbde8a1b5
https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/renderer_context_menu/render_view_context_menu.cc;l=1118;drc=f4096571c202d7aa7e5c9d9b8dc9edebbbeb631e

Therefore, the CL preserves the current non-mullhet behaviour.

Bug: 321678141
Change-Id: Ic6801f45f8d78967983fc98484967240e1766ca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5616371
Commit-Queue: Theo Cristea <theocristea@google.com>
Reviewed-by: Bruno Braga <brunobraga@google.com>
Cr-Commit-Position: refs/heads/main@{#1316491}