[css-flex] Try a more conservative intrinsic sizing algorithm

Our first attempt at a better algorithm was not compatible with the web.
This version gives behavior closer to what is currently shipping in all
browsers.

The difference in this version affects only single-line row containers'
min-content sizes. For such containers where no items are growing from
their flex basis to their desired min contribution, an item's final
contribution to the container's min-content size is (1) its flex-basis
if the item has a 0 flex factor in whichever direction it wants to go
(shrink if flex-basis > min-content contribution, grow otherwise);
otherwise: (2) its min-content contribution.

This will probably only be enabled for a few days, but will give us good
compat data even so.

Bug: 240765
Change-Id: Ib8ae101ec88b4f0e543c8f4ecb713edb68074cb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4602419
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1155331}