AXURL not being set in accessibility tree

[nwtn]

AXURL is null, instead of the src

[yoavweiss]

Shouldn't we spec this as part of picture's accessibility model?

[nwtn]

Perhaps. I don't know much about this stuff, so I assumed these were standard things that weren't element-dependant, and which were spec'ed elsewhere.

ResponsiveImagesCG/picture-element#34