CfC: Republish MIX as CR; deadline July 29th.

This is a CfC to take the following version of MIX back to CR for another
month:

https://w3c.github.io/webappsec/specs/mixedcontent/published/2015-08-CR.html

Based on feedback during the review period for MIX, we've made two
substantive changes to the document since the CR we published in February:

1. We dropped the "Deprecated TLS-protection" concept which was marked as
"at-risk" in the original CR.

2. As discussed in
https://lists.w3.org/Archives/Public/public-webappsec/2015Jul/0137.html,
we've changed MIX to handle passthrough `fetch()` requests inside Service
Workers as optionally-blockable if they're triggered from
optionally-blockable Document-generated requests.

Other editorial changes which can be found in their entirety at
https://github.com/w3c/webappsec/commits/master/specs/mixedcontent/index.src.html
.

Also of note: we now have substantial test coverage at
https://github.com/w3c/web-platform-tests/tree/master/mixed-content. We'll
need to add tests for the Service Worker behavior we expect to see now, but
there's already very solid overlap in behavior between Chrome, Firefox,
Safari, and IE, such that I hope advancement to PR will be relatively
straightforward.

The deadline for this CfC is one week from today, July 29th. As always,
explicit feedback to public-webappsec@w3.org is appreciated. :)

Thanks!

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

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