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

third_party/WebKit/LayoutTests/fast/encoding/api/ascii-supersets-expected.txt

 This is a testharness.js-based test.
+PASS ASCII superset encoding: ibm866 
 PASS ASCII superset encoding: iso-8859-2 
 PASS ASCII superset encoding: iso-8859-3 
 PASS ASCII superset encoding: iso-8859-4 

third_party/WebKit/LayoutTests/fast/encoding/api/resources/shared.js

@@ -7,7 +7,6 @@
 //   * whitespace
 //   * encodings.json data assigned to `encodings_table` (for tests)
 //   * UTF encodings listed in `utf_encodings` (for tests)
-//   * 'ibm866' not yet supported - crbug.com/277023
 //   * 'gb18030' distinct from 'gbk' - crbug.com/339862
 //   * 'big5-hkscs' distinct from 'big5' - crbug.com/277040
 //   * 'hz-gb-2312' is replacement label - w3.org/Bugs/Public/show_bug.cgi?id=25339
@@ -31,7 +30,6 @@ var encodings_table = [
     },
     {
         "encodings": [
-            /* FIXME: Support ibm866 (crbug.com/277023)
             {
                 "labels": [
                     "866",
@@ -41,7 +39,6 @@ var encodings_table = [
                 ],
                 "name": "ibm866"
             },
-             */
             {
                 "labels": [
                     "csisolatin2",