Canceled torrent does not cancel partially downloaded file

[btlechowski]

Steps to Reproduce

1. Go to https://webtorrent.io/free-torrents
2. Open Big Buck Bunny magnet link
3. Click 'Start Torrent'
4. Save Big Buck Bunny.mp4 file
5. Download around 10% of the file and click Stop Downloading on torrent page
   ALTERNATE CASE:
   5 Download around 10% and close the tab with the torrent

Actual result:

Brave dumps partially downloaded Big Buck Bunny.mp4 to disk and marks it as properly downloaded.
The file is saved as Big Buck Bunny.mp4, even though it is partially downloaded.

Expected result:

The file being downloaded is cancelled. Just like it is cancelled when user exits the browser.

Reproduces how often:

always

Brave version (chrome://version info)

Verification passed on

Brave	0.55.5 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Windows 7

cc @yrliou @bridiver

Related issues: #289 #855