Prevent transitioncancel from firing after transtionend

Per spec (https://drafts.csswg.org/css-transitions-2/#event-dispatch)
a transitioncancel should be fired on the following phase change

 not idle and not after --> idle

Previously we were only test for not idle --> idle, meaning that a
finished transition could erroneously trip a transitioncancel.

Bug: 979556
Change-Id: Iecd6c8d2a7ca4564925910f092fca4c78b6d79da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2348416
Commit-Queue: Kevin Ellis <[email protected]>
Reviewed-by: Xida Chen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#796973}

css/css-transitions/event-dispatch.tentative.html

@@ -414,4 +414,22 @@
   await waitForAnimationFrames(2);
 }, 'Cancel the transition after clearing the target effect');
 
+promise_test(async t => {
+  const { transition, watcher, div } = setupTransition(t, 'margin-left 100s');
+
+  // Seek to After phase.
+  transition.finish();
+  const events = await watcher.wait_for(
+    ['transitionrun', 'transitionstart', 'transitionend'],
+    {
+      record: 'all',
+    }
+  );
+
+  transition.cancel();
+
+  // Then wait a couple of frames and check that no event was dispatched
+  await waitForAnimationFrames(2);
+}, 'Cancel the transition after it finishes');
+
 </script>