Remove stray comment about ConvertableToTraceFormat

The functions have been converted to ConvertableToTraceFormat
since this comment was introduced but it wasn't removed.

Review URL: https://codereview.chromium.org/423943004

git-svn-id: svn://svn.chromium.org/blink/trunk@179065 bbb929c8-8fbe-4397-9dbb-9b2b20218538