Added method to reset Translate preferences.

Mobile needs a way to reset the Translate preferences.

BUG=None

Review URL: https://codereview.chromium.org/71353005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235126 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235126