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If the recommended locales for a public session are invalid/unavailable,
public sessions should default to the current UI language, not the
locale that happens to be first in alphabetic order (Arabic for Chrome OS
running in English).

BUG=403550
TEST=Extended unit and browser tests

Review URL: https://codereview.chromium.org/483523005

Cr-Commit-Position: refs/heads/master@{#291408}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291408 0039d316-1c4b-4281-b951-d872f2087c98