Property on fetchEvent to indicate the target client

[jakearchibald]

The intent is to allow developers to identify a client that will be replaced by a given navigation. The developer will be able to identify a reload by looking at the client id, its url, the request url and the target client id.

[annevk]

This will need some careful integration with HTML's navigate algorithm.

[jungkees]

With the work done through #870, now we have all of them: the client id, its url, the request url and the target client id. Closing.