Fix race condition between account picker and GAIA sign-in screen
SigninScreenHandler determines whether the account picker or the GAIA
sign-in screen should be shown. However, the GAIA sign-in screen shows
asynchronously, which causes a race condition. The GAIA sign-in screen
may finish showing after the SigninScreenHandler has decided to switch
to the account picker instead.
This race is causing two DeviceLocalAccountTest.* tests to flake on the
Chrome OS ASan/LSan bots.
BUG=432119
TEST=DeviceLocalAccountTest.* no longer flakes
Review URL: https://codereview.chromium.org/
718623003
Cr-Commit-Position: refs/heads/master@{#303656}