Allow top-level frames to request any USB device.

device/usb/mock_usb_service.cc

Index: device/usb/mock_usb_service.cc
diff --git a/device/usb/mock_usb_service.cc b/device/usb/mock_usb_service.cc
index e1d98c131a2c15dcf6c635c2783d10bb8fe02ebe..ff78d42f479bfc68e7c685d95e515886548ffb5a 100644
--- a/device/usb/mock_usb_service.cc
+++ b/device/usb/mock_usb_service.cc
@@ -7,11 +7,16 @@
 #include <string>
 #include <vector>
 
+#include "base/threading/thread_task_runner_handle.h"
 #include "device/usb/usb_device.h"
 
 namespace device {
 
-MockUsbService::MockUsbService() : UsbService(nullptr, nullptr) {}
+MockUsbService::MockUsbService()
+    : UsbService(base::ThreadTaskRunnerHandle::IsSet()
+                     ? base::ThreadTaskRunnerHandle::Get()
+                     : nullptr,
+                 nullptr) {}
 
 MockUsbService::~MockUsbService() {
   // Shutdown() must be called before the base class destructor.