Remove incorrect comment for createXHRTextDecoder

createXHRTextDecoder never returns null.
Removed the incorrect comment and make the createXHRTextDecoder
implementation clear.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23537047

git-svn-id: svn://svn.chromium.org/blink/trunk@158115 bbb929c8-8fbe-4397-9dbb-9b2b20218538