Steps to reproduce:

1. Go to http://w3c-test.org/presentation-api/controlling-ua/startNewPresentation_sandboxing_success-manual.html
2. Click "Start Presentation Test" and select a device

Expected:
Start presentation on selected device

Actual:
Fail to start presentation on selected device. Test fails:

assert_equals: Presentation could not be started from nested frame. expected "success" but got "UnknownError"
    at Test.<anonymous> (https://web-platform.test:8443/presentation-api/controlling-ua/startNewPresentation_sandboxing_success-manual.html:24:17)
    at Test.step (https://web-platform.test:8443/resources/testharness.js:1406:25)
    at https://web-platform.test:8443/resources/testharness.js:1430:35