Service Worker: Remove NYI methods from IDL for easier polyfilling

Rather than exposing methods that throw/reject with "not implemented",
simply elide them from the IDL. This makes it easier to write polyfills
that fill in the missing methods.

BUG=428232
R=jkarlin@chromium.org

Review URL: https://codereview.chromium.org/690723002

git-svn-id: svn://svn.chromium.org/blink/trunk@184603 bbb929c8-8fbe-4397-9dbb-9b2b20218538