Encoding API: Enable test of ibm866

Following Chromium r284167 the "ibm866" encoding is supported and
we can enable the TextDecoder test for it.

R=jshin@chromium.org
BUG=277023

Review URL: https://codereview.chromium.org/403793003

git-svn-id: svn://svn.chromium.org/blink/trunk@178502 bbb929c8-8fbe-4397-9dbb-9b2b20218538