Add getUILanguage() to chrome.i18n which returns the browser UI language.

For the moment, extension and internal code relies on navigator.language
to return the browser UI language but it should return the preferred
language for fingerprinting reasons, see http://crbug.com/101138.

BUG=348402

Review URL: https://codereview.chromium.org/185423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255663 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255663