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There was a potential for a subtle race built into these tests:
if the timing of determining the language shifted, the observer
may end up waiting forever. This is what's happening in
https://codereview.chromium.org/1398823004, and the race
becomes visible.

Instead, let's reogranize observation of
chrome::NOTIFICATION_TAB_LANGUAGE_DETERMINED to be managed
at the test harness level and ensure that observation begins
outside of the message loop pump.

Also, let's move a very similar test from browser_browsertest
and convert it to use the same machinery.

Finally, refactor that test to remove another subtle race:
it actually opens two tabs and receives two notifications,
but that is only vaguely implied by the test logic.

R=jam,jochen,toyoshim@chromium.org
BUG=521166

Review URL: https://codereview.chromium.org/1420093004

Cr-Commit-Position: refs/heads/master@{#357176}