Make InputDeviceInfo.getCapabilities() return empty for cameras that are unavailable to the UA because they are opened in exclusive mode by other applications. The only platform affected in practice is Windows, which is the only platform that exhibits the issue.
Allows developers to avoid expensive getUserMedia() calls that are known in advance to fail by knowing the capabilities of a device. If the device is unavailable, its capabilities are reported as empty, so an application might prefer not to call getUserMedia in this case.