unit_tests: Restore locale to en-US to fix flaky tests.

OmniboxPedals.DataLoadsForAllLocales loads a bunch of locales to the
global shared resource bundle, and does not restore en-US. This breaks
downstream tests who expect English strings.

The fix is to make unit_tests suite ensure that the loaded locale is
en-US before each test runs.

Bug: 956719
Change-Id: I8e407b4956f4f0314b7d045217fe0008c77d9c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2678132
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#852034}