Throw TypeError if easing string is invalid.

third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html

Index: third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html
diff --git a/third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html b/third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html
index 769a70c0e28a832618d5067c988ca8cd4d7bad14..29c56676abdc98d02a9bf4a56d1fc88714da2e3d 100644
--- a/third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html
+++ b/third_party/WebKit/LayoutTests/animations/function-easing-use-counters-linear.html
@@ -11,12 +11,19 @@ test(() => {
   document.documentElement.animate([], { });
   document.documentElement.animate([], { easing: 'linear' });
   document.documentElement.animate([], { easing: 'step-start' });
-  document.documentElement.animate([], { easing: 'invalid' });
   assert_false(internals.isUseCounted(document, WebAnimationsEasingAsFunctionLinear));
   assert_false(internals.isUseCounted(document, WebAnimationsEasingAsFunctionOther));
 }, 'Non-function values for easing are not use-counted.');
 
 test(() => {
+  assert_throws(
+      {name: 'TypeError'},
+      function() { document.documentElement.animate([], { easing: 'invalid' }) });
+  assert_false(internals.isUseCounted(document, WebAnimationsEasingAsFunctionLinear));
+  assert_false(internals.isUseCounted(document, WebAnimationsEasingAsFunctionOther));
+}, 'Invalid non-function values for easing are not use-counted.');
+
+test(() => {
   // This linear function value is the one expected from uses of the unpatched
   // Web Animations polyfill (i.e. old versions lacking
   // https://github.com/web-animations/web-animations-next/pull/423).