Assets 3
Re-resubmit cr/1261477

The issue was that there is a race condition between the abort going
through, and the downloaded file being persisted on disk. I added an
extra check in the WPT test.

Change-Id: I98aa005b4637c08f60a126a6c7c896c1c0edfddf
Reviewed-on: https://chromium-review.googlesource.com/c/1276557
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598848}