Add a feature flag for the new Windows USB backend.

device/usb/usb_service_impl.cc

Index: device/usb/usb_service_impl.cc
diff --git a/device/usb/usb_service_impl.cc b/device/usb/usb_service_impl.cc
index 2f5883b0678aababf05afc3b261d5111b8ae4546..6c1922d37c1dd5561ccaf1d2b71e42e19a289728 100644
--- a/device/usb/usb_service_impl.cc
+++ b/device/usb/usb_service_impl.cc
@@ -19,7 +19,6 @@
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "components/device_event_log/device_event_log.h"
 #include "device/usb/usb_device_handle.h"
@@ -215,17 +214,17 @@ void OnDeviceOpenedReadDescriptors(
 }  // namespace
 
 UsbServiceImpl::UsbServiceImpl(
-    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner)
-    : UsbService(base::ThreadTaskRunnerHandle::Get(), blocking_task_runner),
+    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_in)
+    : UsbService(std::move(blocking_task_runner_in)),
 #if defined(OS_WIN)
       device_observer_(this),
 #endif
       weak_factory_(this) {
-  blocking_task_runner->PostTask(
-      FROM_HERE, base::Bind(&InitializeUsbContextOnBlockingThread,
-                            base::ThreadTaskRunnerHandle::Get(),
-                            base::Bind(&UsbServiceImpl::OnUsbContext,
-                                       weak_factory_.GetWeakPtr())));
+  blocking_task_runner()->PostTask(
+      FROM_HERE,
+      base::Bind(&InitializeUsbContextOnBlockingThread, task_runner(),
+                 base::Bind(&UsbServiceImpl::OnUsbContext,
+                            weak_factory_.GetWeakPtr())));
 }
 
 UsbServiceImpl::~UsbServiceImpl() {