[Autofill] Use ShadowDOM placeholder to preview suggestions.

The first patch is a re-upload of
https://chromium-review.googlesource.com/c/chromium/src/+/646754

The follow-up patches will add some modifcations on how we preview
username and password suggestions.

The suggestions will be in black text, and the password suggestions
should be hidden behind dots.

Bug: 753645
Change-Id: I1d28ea47f443fc40a1cddf2cdef6b1ec86c4491e
Tbr: tkent@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/702056
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509961}
20 files changed