Bug 1575407 [wpt PR 18573] - [css-flexbox] Only clear the override size if we're doing layout, a=testonly

Automatic update from web-platform-tests
[css-flexbox] Only clear the override size if we're doing layout

And make ChildUnstretchedLogical{Width,Height} handle the case where
an override size is set (by clearing and re-setting).

The problem with clearing the override size, but not doing layout,
is that we may later do a simplified layout which will then
lay out as shrink-to-fit, which is undesired.

Bug: 992010
Change-Id: I18d3ae66f62cbdc9cdcf76810dc0eff6be915af6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758940
Commit-Queue: Christian Biesinger <cbiesingerchromium.org>
Auto-Submit: Christian Biesinger <cbiesingerchromium.org>
Reviewed-by: David Grogan <dgroganchromium.org>
Reviewed-by: Morten Stenshorne <mstenshochromium.org>
Reviewed-by: Emil A Eklund <eaechromium.org>
Cr-Commit-Position: refs/heads/master{#689485}

--

wpt-commits: 768d6294ed324158d4269a833fda94ea7927efe3
wpt-pr: 18573

UltraBlame original commit: ffeb51048e9b089ec1a2899bbf11dbcca798b201

Author: marco-c