Add WICG specs with ongoing development in browsers

[dontcallmedom]

Compared to #86, these implementations aren't quite public facing yet - I think they still match the criteria of implementors interest, but I thought their addition would benefit from separate explicit review.

https://wicg.github.io/content-index/spec/ from WICG/content-index
origin trial in Chrome https://www.chromestatus.com/features/5658416729030656
https://wicg.github.io/custom-state-pseudo-class/ from WICG/custom-state-pseudo-class
implemented in chrome behind a flag https://www.chromestatus.com/features/6537562418053120
https://wicg.github.io/overscroll-scrollend-events/ from WICG/overscroll-scrollend-events
behind a flag per https://www.chromestatus.com/features/5650553247891456
https://wicg.github.io/savedata/ from WICG/savedata
behind a flag https://www.chromestatus.com/features/5076813940260864
https://wicg.github.io/webpackage/loading.html
origin trial in Chrome https://www.chromestatus.com/features/5126805474246656
https://wicg.github.io/is-input-pending/ from WICG/is-input-pending in development in Chrome per https://www.chromestatus.com/features/5719830432841728
https://wicg.github.io/local-font-access/ from WICG/local-font-access in development in Chrome per https://www.chromestatus.com/features/6234451761692672
https://wicg.github.io/webhid/ from WICG/webhid in development in Chrome per https://www.chromestatus.com/features/5172464636133376

[tidoust]

That looks good to me but it may be worth checking with @foolip as these will create IDL extracts down the road.

[dontcallmedom]

friendly ping @foolip

[foolip]

I've gone through each addition and checked, they all seem like live proposals. There will probably be some issue with some of these, but let's land them and find out :)