Change name from targetClientId to replacesClientId (#1333)
Change name from targetClientId to replacesClientId (#1333)

We had decided (and I forgot) to change the name of FetchEvent.targetClientId to
.replacesClientId to clarify the meaning that this client is a to-be-replaced
client: https://github.com/w3c/ServiceWorker/issues/1091#issuecomment-342569083.

Fetch PR: https://github.com/whatwg/fetch/pull/774.
HTML PR: https://github.com/whatwg/html/pull/3788.

Related issue: https://github.com/w3c/ServiceWorker/issues/1245.
slightlyoff/ServiceWorker,slightlyoff/ServiceWorker