First, read the following files to familiarize yourself with browser test APIs:

• //content/public/test/browser_test_base.h
• //content/public/test/browser_test_utils.h
• //content/public/test/content_browser_test.h

Next, read the following MDN articles to familiarize yourself with WebUSB, a JavaScript API for interacting with USB devices:

• https://developer.mozilla.org/en-US/docs/Web/API/WebUSB_API
• https://developer.mozilla.org/en-US/docs/Web/API/USB
• https://developer.mozilla.org/en-US/docs/Web/API/USBDevice

Finally, add a test case to //content/browser/usb/usb_browsertest.cc that tests the following scenario:

1. Open the mock device with open()
2. Check that the device's opened attribute is true
3. Close the mock device with close()
4. Check that the device's opened attribute is now false
5. Check that the device is still in the array returned by navigator.usb.getDevices()

Once the test is written, verify the test passes with autotest.py.

There's no need to modify any .gn files because usb_browsertest.cc is an existing source file.