ExtendableEvent and InstallEvent should have an EventConstructor

This brings the events in line with the Service Worker specification.

This allows us to implement events deriving from ExtendableEvent
(most notably Push, NotificationClick and NotificationError) according
to their specifications as well.

BUG=420896

Review URL: https://codereview.chromium.org/715013003

git-svn-id: svn://svn.chromium.org/blink/trunk@185365 bbb929c8-8fbe-4397-9dbb-9b2b20218538