[LayoutNG] Don't always make a post-flexed main size definite

It should only be definite if the element already had a definite main
size or if the container has a definite main size.

This is #2 from https://drafts.csswg.org/css-flexbox/#definite-sizes

Bug: 845235
Change-Id: I0230080d22ada93ebc8bae09aeda629d87cf5b6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1797442
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698790}

css/css-flexbox/percentage-heights-003.html

@@ -76,6 +76,15 @@
     </div>
 </div>
 
+<!-- indefinite unwrapped column flexbox where item doesn't stretch -->
+<div style="height: 100px;">
+    <div class="flexbox column">
+        <div style="align-self: flex-start">
+            <span data-expected-height="0"></span>
+        </div>
+    </div>
+</div>
+
 <!-- indefinite wrapped column flexbox -->
 <div style="height: 100px;">
     <div class="flexbox column-wrap">