Reland "Reland "Do not reveal username value till user gesture.""

This is a reland of 3d4d84928567f74d93d951932eafa1b3d779923b
Original change's description:
> Reland "Do not reveal username value till user gesture."
>
> Original change's description:
> > Passwords are not available for a page till user gesture.
> > But for usernames it was not implemented. This patch implements
> > this: use SetSuggestedValue for username and register
> > username_element to gatekeeper, which manages revealing
> > the input suggested value to JS after user action.
> >
> > Along the way, a small improvement was implemented, setting
> > the username value only in one place which simplifies the code.
>
> > Bug: 798492
> > Change-Id: Ib37637dd861f44b94674c6d0060ebc91dfd73cff
> > Reviewed-on: https://chromium-review.googlesource.com/850394
> > Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
> > Reviewed-by: Dominic Battré <battre@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#527598}
>
> Original Patch is PatchSet 1.
> Flakiness was fixed on CL https://chromium-review.googlesource.com/c/chromium/src/+/859960
> This CL differs from original only in small compilation fix in PatchSet 5 (compilation
> errors were because of changes in another CL).
>
> TBR=battre@chromium.org
>
> Bug: 798492
> Change-Id: Ife2f67ca2cd75372ef388d077acb9d53b47be92d
> Reviewed-on: https://chromium-review.googlesource.com/854173
> Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
> Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#528593}

Flakiness that was a reason of last revert was fixed on
https://chromium-review.googlesource.com/c/chromium/src/+/860649

TBR=battre@chromium.org

Bug: 798492
Change-Id: Ia277559ebe541da25d21f493d0ae2889c676aa9c
Reviewed-on: https://chromium-review.googlesource.com/862143
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528716}
5 files changed