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Long overdue tests. Added for https://crrev.com/c/2315728.
Done after polymer3 migration of os_languages_page at
https://crrev.com/c/2308549.

Bug: 1106033
Change-Id: Ibaa2fca8c2ba4def0b4da8287dc984ac1a8d4b09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2345926Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: My Nguyen <myy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796770}