[css-grid] Fix accidental regression where min-size is not considered for stretching auto tracks. Fixes #1866.

Author: tabatkins

css-grid-1/Overview.bs

@@ -4059,6 +4059,9 @@ Stretch <css>auto</css> Tracks</h3>
 	an ''grid-template-columns/auto'' <a>max track sizing function</a>
 	by dividing any remaining positive, <a>definite</a> <a>free space</a>
 	equally amongst them.
+	If the <a>free space</a> is <a>indefinite</a>,
+	but the <a>grid container</a> has a <a>definite</a> <a property lt=min-width>min-width/height</a>,
+	use that size to calculate the <a>free space</a> for this step instead.
 
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