Use Web IDL Union types in Service Worker's Cache interface

No more overloads! The method/methodImpl pattern could be further
collapsed, but this seems like a good start while we work out kinks.

The 'RequestInfo' typedef is from the fetch spec[1], also used in the
ServiceWorker spec[2] to improve readability.

[1] https://fetch.spec.whatwg.org/
[2] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html

BUG=240176
R=bashi@chromium.org,jkarlin@chromium.org

Review URL: https://codereview.chromium.org/700323003

git-svn-id: svn://svn.chromium.org/blink/trunk@185073 bbb929c8-8fbe-4397-9dbb-9b2b20218538