Fix crash during serialization of the font shorthand.

It's not clear whether calc() expressions should be converted into
font keywords; not doing it seems like the most consistent handling,
given that calc() is never converted into literals anywhere else,
but it clashes with existing practice.

Bug: 40946458
Change-Id: I60c4fa354ec016ceeb872b7dabbef14d69412814
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6057817
Commit-Queue: Steinar H Gunderson <[email protected]>
Reviewed-by: Anders Hartvoll Ruud <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1389776}

css/css-fonts/font-shorthand-serialization-font-stretch.html

@@ -84,4 +84,18 @@
     assert_equals(div.style.fontStretch, "50.01%");
     assert_equals(div.style.font, "");
 }, "Percentages which cannot be transformed into keywords should prevent the font shorthand from serializing, but not fontStretch");
+
+test(function() {
+    const div = document.getElementById("test");
+
+    // This isn't well-specified, but appears to be consistent across browsers.
+    div.style.fontStretch = "calc(50%)";
+    assert_equals(div.style.fontStretch, "calc(50%)");
+    assert_equals(div.style.font, "ultra-condensed medium serif");
+
+    div.style.fontStretch = "calc(50% + 25% * sign(100em - 1px))";
+    assert_equals(div.style.fontStretch, "calc(50% + 25% * sign(100em - 1px))");
+    assert_equals(div.style.font, "");
+}, "calc() transformation into keywords");
+
 </script>