Clean up callback interface code and template

The callback interface code was the first code and template we wrote.
Since then we've improved style and template usage; this simplifies
the code and brings it up-to-date.

TBR=haraken
BUG=345503

Review URL: https://codereview.chromium.org/215993002

git-svn-id: svn://svn.chromium.org/blink/trunk@170254 bbb929c8-8fbe-4397-9dbb-9b2b20218538