Fix debug crash when compositing keyword animations together

third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html

Index: third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html
diff --git a/third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html b/third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..2611fdb89dad6f262238be05a307253dc7116060
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html
@@ -0,0 +1,10 @@
+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<div id="target"></div>
+<script>
+test(() => {
+  target.animate({position: 'absolute'}, {fill: 'forwards'});
+  target.animate({position: 'static', composite: 'add'}, {fill: 'forwards'});
+  getComputedStyle(target).position;
+}, "Don't crash when compositing keyword animations.");
+</script>