webusb support

[mrose17]

Test Case:

• ensure that navigator.usb doesn't return undefined via the DevTools as per Re-enable webusb feature brave-core#2574
• ensure that you can connect either a Ledger Nano S or a Trevor device via https://mycrypto.com/

Original Issue

sadly, webusb appears to be table stakes...

Description

The browser does not support webusb.

Steps to Reproduce

1. Connect to a service that requires webusb, e.g., beta.shapeshift.com
2. Attempt to pair your KeepKey

Actual result:

Expected result:

Success

Reproduces how often:

100%

Brave version (brave://version info)

Version 0.64.77 Chromium: 74.0.3729.169 (Official Build) (64-bit)```

## Version/Channel Information:
- Can you reproduce this issue with the current release? yes
- Can you reproduce this issue with the beta channel? yes
- Can you reproduce this issue with the dev channel? yes
- Can you reproduce this issue with the nightly channel? yes

## Other Additional Information:
- Does the issue resolve itself when disabling Brave Shields? yes
- Does the issue resolve itself when disabling Brave Rewards? yes
- Is the issue reproducible on the latest version of Chrome? no, it supports webusb

## Miscellaneous Information:
None.

[simonhong]

Brave returns undefined for navigator.usb.

[kjozwiak]

Similar findings under #3120 (comment) when investigated why Trezor wasn't working. Will close off #3120 once this has been verified. You would receive the following error:

iframe.9b57ae6d349fd314f512.js:8 [trezor-link] Error in WebUsbPlugin.init Error: WebUSB is not available on this browser.

[kjozwiak]

Verification PASSED on macOS 10.14.5 x64 using the following build:

Brave	0.66.88 Chromium: 75.0.3770.87 (Official Build) beta(64-bit)
Revision	9dc58a2353af60ab2b48bab98a25bc43ed59085d-refs/branch-heads/3770@{#982}
OS	Mac OS X

• ensured that navigator.usb isn't returning undefined
• connected to https://connect.trezor.io with the Trezor and updated the FW without any issues
• connected to https://mycrypto.com with the Trezor without any issues
• connected to https://mycrypto.com with the Ledger Nano S without any issues

Verification passed on

Brave	0.66.98 Chromium: 75.0.3770.100 (Official Build) beta(64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Linux

• Verified Trezor was updated to new firmware

• Verified Terzor was detected and worked on https://mycrypto.com
  

• Verified Ledger Nano was detected on https://mycrypto.com
  

Verification passed on

Brave	0.66.98 Chromium: 75.0.3770.100 (Official Build) beta(64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified navigator.usb didn't retrun undefined
  

• Verified Trezor worked on mycrypto.com

• Verified Trezor worked with wallet.trezor.com
  

• Verified Ledger Nano S worked with mycrypto.com