[MD settings] I18nBehavior return textContent from i18n()

This CL removes a prior quick-fix that swapped   for \u00a0 and
instead returns the textContent from i18n(). The result (as mentioned
in the updated comments) is that i18n() should be used for non-HTML and
i18nAdvanced() should be used for innerHTML.

BUG=731289
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2927213002
Cr-Commit-Position: refs/heads/master@{#479197}
7 files changed