[Extensions] Re-enable Downloads API test that was disabled on
ChromeOS.

This test passed without flakes for 3000 iterations on the
release build. It failed once out of 3000 iterations on the
debug build. I'll monitor the debug build for flakes and disable
it there if necessary.

Bug: 396510
Change-Id: I7f23603aeb17f93f80a1e3f99863bc1322b423fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2203263
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769769}