Clean up callback interface code and template

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170254

[Nils Barth (inactive), 2014-03-28 01:27:07]

Quick clean-up.

[Nils Barth (inactive), 2014-03-28 01:27:11]

The CQ bit was checked by nbarth@chromium.org

[commit-bot: I haz the power, 2014-03-28 01:27:16]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nbarth@chromium.org/215993002/1

[haraken, 2014-03-28 01:29:27]

LGTM

[commit-bot: I haz the power, 2014-03-28 01:53:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-28 01:53:32]

Try jobs failed on following builders:
  tryserver.blink on win_blink_compile_dbg

[Nils Barth (inactive), 2014-03-28 02:29:48]

The CQ bit was checked by nbarth@chromium.org

[commit-bot: I haz the power, 2014-03-28 02:30:23]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nbarth@chromium.org/215993002/1

[commit-bot: I haz the power, 2014-03-28 02:52:36]

Change committed as 170254