[fetch] Add a bit to Opaque Responses to distinguish redirects (#79)

(apologies if this is a dupe)

@sirdarckcat has brought up an issue (documented here: http://sirdarckcat.blogspot.de/2015/05/service-workers-secure-open-redirect.html ) that seems like it'd be mostly solved through the addition of a single bit to Response objects which notes if the content is the result of a redirect.

At the webappsec f2f today in Berlin, the group there seems to agree that exposing this bit on Responses doesn't leak any new information; it's observable via CSP + iframes already.

/cc @jakearchibald @jungkees @annevk @metromoxie @hillbrad

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https://github.com/whatwg/fetch/issues/79