Encoding API: Use encodings table from spec in tests; add labels tests

BUG=243354
R=kbr@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23021013

git-svn-id: svn://svn.chromium.org/blink/trunk@156501 bbb929c8-8fbe-4397-9dbb-9b2b20218538