cros: Fix WebviewLoginTest.Basic multiple login attempts.

cros: Fix WebviewLoginTest.Basic multiple login attempts.

ShowSigninScreenForCreds is used to automatically fill-in sign-in
forms and submit it. WebviewLoginTest.Basic does that manually
thus should not call it. Otherwise, /_/embedded/signin/challenge
is called multiple times with wrong email (empty) and could trigger
FakeGaia failures.

BUG=472095
Committed: https://crrev.com/5a0107a25031f3ad52a5afa15acbb4fac4560d34
Cr-Commit-Position: refs/heads/master@{#324353}

[xiyuan, 2015-04-08 22:38:45]

xiyuan@chromium.org changed reviewers:
+ dpolukhin@chromium.org

[Dmitry Polukhin, 2015-04-09 01:56:14]

lgtm

[xiyuan, 2015-04-09 02:56:47]

The CQ bit was checked by xiyuan@chromium.org

[commit-bot: I haz the power, 2015-04-09 02:58:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1069423004/1

[commit-bot: I haz the power, 2015-04-09 03:42:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-09 03:43:20]

Patchset 1 (id:??) landed as
https://crrev.com/5a0107a25031f3ad52a5afa15acbb4fac4560d34
Cr-Commit-Position: refs/heads/master@{#324353}