Add integrity values to srcset

[yoavweiss]

As part of the Sub-Resource integrity spec, the issue of integrity checks for image resources came up: https://www.w3.org/Bugs/Public/show_bug.cgi?id=27746

Can we take a stab at baking something like that into srcset?

[yoavweiss]

/cc @mikewest @tabatkins @zcorpan

[zcorpan]

The comments suggest it's too early.

[styfle]

What's the status on SRI?

[zcorpan]

integrity exists on link and script only currently.

https://html.spec.whatwg.org/multipage/semantics.html#attr-link-integrity
https://html.spec.whatwg.org/multipage/scripting.html#attr-script-integrity

http://caniuse.com/#search=integrity shows implementation status.

I'll let @mikewest comment on whether there are other developments with SRI.