Add tor switch option in settings

[simonhong]

Test plan

See brave/brave-core#4046

Description

That setting can enable/disable tor on runtime.
After changing that option, new option will be in effect after restarting.

[btlechowski]

Verification passed on

Brave	1.4.84 Chromium: 80.0.3987.87 (Official Build) beta (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#4046
Tor enabled by default

Tor off

Task manager after closing Tor window

Verification passed on

Brave	1.4.84 Chromium: 80.0.3987.87 (Official Build) beta (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified test plan from Add setting option for disabling tor component brave-core#4046
  Tor enabled by default - When Private window with Tor is ON New private window with tor option is available in Normal and Private tab app menu
  
  

Tor off - When Private window with Tor is off New private window with tor option is not available in Normal and Private tab app menu

No tor utility process is running in task manager when TOR option is disabled and closed all tor tabs

Verified passed with

Brave	1.4.85 Chromium: 80.0.3987.87 (Official Build) beta (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}
OS	macOS Version 10.14.6 (Build 18G103)

• Verified test plan from Add setting option for disabling tor component brave-core#4046
• In addition to the test plan above, confirmed the following items when Tor is off in brave://settings/extensions
  • the 'New Private Window with Tor' option does not show in the hamburger menu on Normal, Private, and Guest windows.
  • keyboard shortcut for Tor window (cmd+opt+N on macOS) does not open a Tor window.
  • New Private Window with Tor is removed from context menus when right clicking on a link

Tor toggle off - When Private Window with Tor is off New Private Window with Tor option is not available in Normal, Private and Guest tab hamburger menus

Confirmed no Tor utility process is running in task manager when Tor option is disabled and all open Tor windows have been closed:

• When Tor is off in brave://settings/extensions it does show on the macOS menu (top of screen). Logged Tor window option shows on macOS File menu when Tor is off in settings #8212 for this.