Implement InstallEvent for ServiceWorker

Most methods are left unimplemented yet.

Also add plumbing to dispatch InstallEvent from chromium
(via ServiceWorkerGlobalScopeProxy).

InstallEvent (and InstallPhaseEvent) is defined in:
https://github.com/slightlyoff/ServiceWorker/blob/master/service_worker.ts#L109

BUG=285976
TEST=to be added in chromium, tested manually locally

Review URL: https://codereview.chromium.org/144943002

git-svn-id: svn://svn.chromium.org/blink/trunk@166006 bbb929c8-8fbe-4397-9dbb-9b2b20218538