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

This reverts commit f69d6349c5e6be22df72d21b1f9f883ac9350870.

Reason for revert: this CL breaks many tests on build bots,
eg. 
https://ci.chromium.org/buildbot/chromium.memory/Linux%20ASan%20Tests%20(sandboxed)/42554 and 
https://uberchromegw.corp.google.com/i/chromium.chromiumos/builders/linux-chromeos-rel/builds/4123

Original change's description:
> 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}

TBR=battre@chromium.org,dvadym@chromium.org

Change-Id: I53dba03674f0fa93f077f51dd481d951f8036825
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 798492
Reviewed-on: https://chromium-review.googlesource.com/852501
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527627}
5 files changed