Move WebServiceWorkerClientsCallbacks definition out of WebServiceWorkerContextClient class

Moving the typedef into WebServiceWorkerClientsInfo as a top-level typedef.
We want to refer to it from modules/* code, so having it in public/web
is problematic.

(Factored out from jsbell's https://codereview.chromium.org/245863003/)

BUG=363965
TEST=interface change only, not yet available

Review URL: https://codereview.chromium.org/247263002

git-svn-id: svn://svn.chromium.org/blink/trunk@172139 bbb929c8-8fbe-4397-9dbb-9b2b20218538