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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

Review URL: https://codereview.chromium.org/1069423004

Cr-Commit-Position: refs/heads/master@{#324353}