Add a feature flag for the new Windows USB backend.

device/usb/usb_service_impl.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "device/usb/usb_service_impl.h"
 
 #include <stdint.h>
 
 #include <list>
 #include <memory>
 #include <set>
 #include <utility>
 
 #include "base/barrier_closure.h"
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #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"
 #include "device/usb/usb_error.h"
 #include "device/usb/webusb_descriptors.h"
 #include "net/base/io_buffer.h"
 #include "third_party/libusb/src/libusb/libusb.h"
 
 #if defined(OS_WIN)
 #include <setupapi.h>
@@ skipping 175 matching lines @@
                                                       device_handle, barrier));
     }
   } else {
     failure_closure.Run();
   }
 }
 
 }  // 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() {
   if (hotplug_enabled_)
     libusb_hotplug_deregister_callback(context_->context(), hotplug_handle_);
   for (auto* platform_device : ignored_devices_)
     libusb_unref_device(platform_device);
 }
 
 void UsbServiceImpl::GetDevices(const GetDevicesCallback& callback) {
@@ skipping 310 matching lines @@
 }
 
 void UsbServiceImpl::EnumerationFailed(PlatformUsbDevice platform_device,
                                        const base::Closure& refresh_complete) {
   libusb_ref_device(platform_device);
   ignored_devices_.insert(platform_device);
   refresh_complete.Run();
 }
 
 }  // namespace device