Avoid mishandling CSSPendingSubstitutionValues in CSS animations

third_party/WebKit/LayoutTests/animations/animate-shorthand-var-crash.html

Index: third_party/WebKit/LayoutTests/animations/animate-shorthand-var-crash.html
diff --git a/third_party/WebKit/LayoutTests/animations/animate-shorthand-var-crash.html b/third_party/WebKit/LayoutTests/animations/animate-shorthand-var-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..1bd15ecedb25fbe821f5e4f16d63247c2790278f
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/animations/animate-shorthand-var-crash.html
@@ -0,0 +1,16 @@
+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<style>
+@keyframes crash {
+  from { border-radius: var(--) }
+}
+#target {
+  animation: crash 1s infinite;
+}
+</style>
+<div id="target"></div>
+<script>
+test(() => {
+  getComputedStyle(target).borderRadius;
+}, 'Do not crash when animating a shorthand with var().');
+</script>