DOM: Fix Observable predicate function

See WICG/observable#140, for the issue about
Predicate having an index parameter.

[email protected]

Bug: 40282760
Change-Id: I94fdfcf9317ec3e364f5636c3808c7a39d88c54e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5478849
Reviewed-by: Joey Arhar <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1291655}

Author: domfarolino

dom/observable/tentative/observable-filter.any.js

@@ -103,3 +103,15 @@ test(() => {
       ['source teardown', 'source abort event', 'filter observable complete']);
 }, "filter(): Upon source completion, source Observable teardown sequence " +
    "happens after downstream filter complete() is called");
+
+test(() => {
+  const source = new Observable(subscriber => {
+    subscriber.next('value1');
+    subscriber.next('value2');
+    subscriber.next('value3');
+  });
+
+  const indices = [];
+  source.filter((value, index) => indices.push(index)).subscribe();
+  assert_array_equals(indices, [0, 1, 2]);
+}, "filter(): Index is passed correctly to predicate");