Animation effect now remains after timeline goes inactive while paused

This issue was fixed in the following CL by changing the calculation
for inherited_time that gets passed to effects in Animation::Update.

https://chromium-review.googlesource.com/c/chromium/src/+/2092188

This CL is adding test coverage for the issue. It has been verified
to fail without the above changes.

Bug: 1082504
Change-Id: I4a15a0dcd23f351fd5fb9e659693d6c803937360
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393059
Commit-Queue: Jordan Taylor <jortaylo@microsoft.com>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805863}

third_party/blink/web_tests/external/wpt/scroll-animations/scroll-animation-effect-phases.tentative.html

@@ -314,4 +314,38 @@ promise_test(async t => {
   );
 }, 'Make scroller inactive, then set current time to an in range time');
 
+promise_test(async t => {
+  const animation = new Animation(
+    createKeyframeEffectOpacity(t),
+    createScrollTimelineWithOffsets(t, "20%", "80%")
+  );
+  const scroller = animation.timeline.scrollSource;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.5 * maxScroll;
+  // allow the scroll to finish.
+  await waitForNextFrame();
+
+  animation.pause();
+  await animation.ready;
+  // verify effect is applied.
+  const expected_progress = 0.5;
+  assert_equals(
+    animation.effect.getComputedTiming().progress,
+    expected_progress,
+    "Verify effect progress after pausing."
+  );
+
+  // cause the timeline to become inactive
+  scroller.style.overflow = 'visible';
+  await waitForAnimationFrames(2);
+  assert_equals(animation.timeline.currentTime, null,
+    'Sanity check the timeline is inactive.');
+  assert_equals(
+    animation.effect.getComputedTiming().progress,
+    expected_progress,
+    "Verify effect progress after the timeline goes inactive."
+  );
+}, 'Animation effect is still applied after pausing and making timeline ' +
+   'inactive.');
+
 </script>
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