Re: [fetch] Redirected range requests and preflights. (#145)

When our media resource loader takes over an HTTP load, it uses the final post-all-redirects URI as its canonical URI for the resource. All subsequent range requests start with that URI; if further redirects occur, they are honoured. The principal(s) associated with the media data are gathered from all final-URIs. If these are different origins that's generally OK: we'll still play the media, though (since  at least one of those origins must not be same-origin with the page) certain APIs will be affected (e.g. after drawing a video frame to a canvas, the canvas will be tainted).

I'm not familiar with the CDN setup described in https://code.google.com/p/chromium/issues/detail?id=532569, but I assume the CDN has a canonical URI which redirects quasi-randomly to one of many mirror URIs, and the mirror URIs never do any more redirects. If so, then by using the final URI from the first load for every subsequent range request we're avoiding any issues.

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