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This is required for cheets_AuthPerf test that waits until Play Store
window is shown. Before we used implicit check that some property file
was created. That was bit flaky due background nature of creating this
file and did not work on all platforms. This CL introduces explicit way
by checking if app is actually shown on Chrome.

BUG=b:110060284
TEST=cheets_AuthPerf passes for Scarlet (where was failing before) + few
other boards.

Change-Id: Ia372b4939853a1c99fdd70a68079e080e14111c3
Reviewed-on: https://chromium-review.googlesource.com/1194735
Commit-Queue: Yury Khmel <khmel@google.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587850}