dillon1000/react

[Fabric] Add `flex: 1` to Offscreen view container (#22019)

Without this style property, the layout of the children is messed up.

The goal is for the container view to have no layout at all. It should
be a completely transparent wrapper, except for when we set `display:
none` to hide its contents. On the web, the equivalent (at least in the
spec) is `display: contents`.

After some initial testing, this seems to be close enough to the desired
behavior that we can ship it. We'll try rolling it out behind a flag.