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Refactor test to newer standards.

Also fixed a race condition in the Welcome screen. In the flow
UpdateLanguageList calls before SetApplicationLocale. But sometimes
results are returned in a different order. That leads to a wrong language
selection (and test flakiness).
This CL cancel UpdateLanguageList request if SetApplicationLocale is
called.

Fixed: 990819
Change-Id: I9117b19ec22e63befb0f904fb91f4fc8e1354c70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209069Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Roman Aleksandrov <raleksandrov@google.com>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770971}