[MIX]: Can we distinguish between images loader via `<picture>`/`srcset` and `<img>`?

It's going to be more or less impossible to begin blocking mixed `<img>`
content by default, given the amount of usage we're likely to see in the
wild (I'm still waiting for metrics from Chrome stable, but I'm not
hopeful). I think it will be significantly less impossible to avoid the
same mistake with the new responsive hotness.

I'd like to distinguish between images loaded via the plain old, boring
`<img>` tag, and images loaded by authors who have opted into The
Future(tm). I believe Brian suggested this in a previous thread, but I
can't find the link at the moment.

I had a quick chat with Yoav this afternoon, and we've got a tiny Blink
patch which changes behavior for `<picture>` and `srcset`. If other vendors
are interested in picking up changes along these lines, I'd love to add it
to the spec.

WDYT?

(CCing Tab and Simon and Yoav for responsive opinions :)).

-mike