This builds on top of #10815, so it's really just the last commit the one that should be reviewed.
I tried to apply the new infrastructure to servo, but failed (for now?).
The problem with it is that it'd require `ThreadSafeLayoutElement` to implement `selectors::Element`, which is a lot of work and might be racy (not totally sure about it though). Thus, I prefered to keep selectors eager until knowing that it's safe to do it.
r? @mbrubeck for style changes, @bholley for the geckolib changes (minimal for now, glue + a list of lazy PEs must be added)
Source-Repo: https://github.com/servo/servo
Source-Revision: 29823cb378ad0b05a82cfdd133c401a678a19007
d72fd6f4ec