Service Workers: InstallPhaseEvent -> ExtendableEvent

The concept of an event that you can call waitUntil() on to delay the
default behavior until the passed Promise resolves was made a more
generic concept in the spec[1], and renamed so it can be used in other
specs in the future.

[1] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#extendable-event-interface

BUG=413662
R=dominicc@chromium.org,tkent@chromium.org

Review URL: https://codereview.chromium.org/588383002

git-svn-id: svn://svn.chromium.org/blink/trunk@182538 bbb929c8-8fbe-4397-9dbb-9b2b20218538