Do not reveal username value till user gesture.

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}
5 files changed