[PR] ServiceWorkerMessageEvent extends ExtendableEvent
[PR] ServiceWorkerMessageEvent extends ExtendableEvent

Summary:
See https://w3c.github.io/ServiceWorker/#service-worker-postmessage
and https://github.com/GoogleChrome/samples/blob/da20dccd06edfb5008bdf04a02950c1b7969198d/service-worker/post-message/service-worker.js#L138-L140
Pull Request resolved: https://github.com/facebook/flow/pull/4858

Reviewed By: dsainati1

Differential Revision: D13855183

Pulled By: nmote

fbshipit-source-id: 871a48fd57597203e12584fe52c180cd1dc03646

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow