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We want the default languages (accept languages for Chrome, preferred languages for ChromeOS) to be blocked so that we don't offer translation by default.
This is achieved by:
(1) Giving a default to translate's blocked language pref when initialized.
(2) Resetting empty blocked languages to the (new) default (to fix current users' prefs).

This changes also means that blocked languages aren't empty in tests anymore, and that translate pref registration now requires string resources to be available.
Hence, a few translate behavior tests needed to be adjusted.

Bug: 902354, 872096
Change-Id: I7fae351d85eca47f68325a2acfe3abc5073ce3c7
Reviewed-on: https://chromium-review.googlesource.com/c/1473431Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: John Wu <jzw@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Commit-Queue: Alexandre Frechette <frechette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635653}