Sending a `context` HTTP request header.

On the last call, we briefly discussed the idea of sending a `context` HTTP
request header containing the request context (
http://fetch.spec.whatwg.org/#concept-request-context) associated with the
request.

That is, a request generated from `<img src="/image.png">` would send
something like `Context: image`, while a request generated from `<script
src="/image.png"></script>` would send `Context: script`. This could
potentially help developers limit their exposure to certain kinds of attack
(e.g. a JSONP endpoint could reject non-"script" requests, which would
mitigate exposure to things like
http://miki.it/blog/2014/7/8/abusing-jsonp-with-rosetta-flash/).

I haven't thought through the impacts of such a header, but I'd like to
make sure the suggestion doesn't stay buried in the service worker thread.
:)

--
Mike West <mkwst@google.com>
Google+: https://mkw.st/+, Twitter: @mikewest, Cell: +49 162 10 255 91

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