CfC: Republish MIX as a CR; deadline Sept. 15th.

The CfC to republish MIX as a CR (
https://lists.w3.org/Archives/Public/public-webappsec/2015Jul/0157.html)
stalled out a bit as Fetch restructured the overloaded `context` concept
that we'd been relying on. I've updated the document to use the new
concepts that have been introduced to take its place (Request's
`initiator`, `type`, `destination`, and `target browsing context`).

With those changes in place, I'd like to publish the following version of
MIX as a new version of the CR:

https://w3c.github.io/webappsec/specs/mixedcontent/published/2015-09-CR.html

Anne and I went back and forth on both the passthrough request definition
and the changes to the algorithms. I believe they're solid enough for
implementation and wide review now.

The deadline for this CfC is one week from today, Sept 15th. As always,
positive feedback to public-webappsec@w3.org is appreciated.

CCing the folks involved in the last thread.

--
Mike West <mkwst@google.com>, @mikewest

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