Re: [streams] Custom tee function (#401)

Because sites will do things like `chunk.processed = true` to communicate between the two sides of the tee() if we return same object.

If we don't spec same object or different object, a browser returning same object may still lead to sites doing this.  Which then breaks the site in other browsers.  I think its better to be explicit.

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https://github.com/whatwg/streams/issues/401#issuecomment-152192668