[css-flexbox-1] Drop justify-content from affecting static-position of abspos within flexbox.

[bfgeek]

From: #7596

The above issue resolved that align-content wont affect the static-position of abspos children within a flexbox.

This issue tracks the same change for justify-content. Currently all browsers support this behaviour.
I've added a use-counter in Blink to determine how web compatible this change would be.

[dholbert]

(Repeating what I said about this proposal in #7596 (comment) when this was initially proposed as part of that issue):

I like this idea in theory (i.e. I like the idea of fully switching to only respect align-self / justify-self for aligning the staticpos of abspos flex children), but I'm a bit skeptical that it'll be web-compatible in the justify axis, given that:
a) all browsers currently respect justify-content for these children (and this proposal would make us stop doing that)
b) no browsers currently respect justify-self for these children (and this proposal would make us start doing that)

Each of (a) and (b) seem like they could cause webcompat problems.

Having said that: if IanK's use-counters prove it to be shippable without breaking the web, it'd be a pretty trivial change for Gecko, implementation-wise.