Only handle torrent URLs when WebTorrent extension is enabled

[yrliou]

We should only handle torrent URLs when WebTorrent extension is enabled as we currently does for magnet URLs.

[btlechowski]

Verification passed on

Brave	0.58.11 Chromium: 71.0.3578.80 (Official Build) beta (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Windows 7 x64

Used STR from brave/brave-core#832

Verification PASSED on Ubuntu 18.04 x64 using the following build:

Brave	0.58.11 Chromium: 71.0.3578.80 (Official Build) beta (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Linux

• went through the STR from Only handle torrent URLs when WebTorrent extension is enabled brave-core#832

Verification PASSED on macOS 10.14 x64 using the following build:

Brave	0.58.14 Chromium: 71.0.3578.98 (Official Build) (64-bit)
Revision	15234034d19b85dcd9a03b164ae89d04145d8368-refs/branch-heads/3578@{#897}
OS	Mac OS X

• went through the STR from Only handle torrent URLs when WebTorrent extension is enabled brave-core#832

[kjozwiak]

@yrliou noticed that when using Open link in private window with Tor, we're still prompting users to open the .torrent files with whatever third party application is associated with .torrent files. I know we basically blocked everything re: .torrent when we were on muon.

Do we know if there's any privacy concerns in allowing .torrent files to be downloaded from a none-Tor Window to a Tor Window?