Move Text Encoding API to 'stable'

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/iWDqDWQ8mhs

BUG=243354
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/418133011

git-svn-id: svn://svn.chromium.org/blink/trunk@178992 bbb929c8-8fbe-4397-9dbb-9b2b20218538