Add a feature flag for the new Windows USB backend.

device/usb/usb_service.cc

 // Copyright 2015 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.h"
 
 #include "base/bind.h"
+#include "base/feature_list.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.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/base/features.h"
 #include "device/usb/usb_device.h"
 
 #if defined(OS_ANDROID)
 #include "device/usb/usb_service_android.h"
 #elif defined(USE_UDEV)
 #include "device/usb/usb_service_linux.h"
 #else
+#if defined(OS_WIN)
+#include "device/usb/usb_service_win.h"
+#endif
 #include "device/usb/usb_service_impl.h"
 #endif
 
 namespace device {
 
 UsbService::Observer::~Observer() {}
 
 void UsbService::Observer::OnDeviceAdded(scoped_refptr<UsbDevice> device) {
 }
 
 void UsbService::Observer::OnDeviceRemoved(scoped_refptr<UsbDevice> device) {
 }
 
 void UsbService::Observer::OnDeviceRemovedCleanup(
     scoped_refptr<UsbDevice> device) {
 }
 
 void UsbService::Observer::WillDestroyUsbService() {}
 
 // static
 std::unique_ptr<UsbService> UsbService::Create(
     scoped_refptr<base::SequencedTaskRunner> blocking_task_runner) {
 #if defined(OS_ANDROID)
   return base::WrapUnique(new UsbServiceAndroid(blocking_task_runner));
 #elif defined(USE_UDEV)
   return base::WrapUnique(new UsbServiceLinux(blocking_task_runner));
-#elif defined(OS_WIN) || defined(OS_MACOSX)
+#elif defined(OS_WIN)
+  if (base::FeatureList::IsEnabled(kNewUsbBackend))
+    return base::WrapUnique(new UsbServiceWin(blocking_task_runner));
+  else
+    return base::WrapUnique(new UsbServiceImpl(blocking_task_runner));
+#elif defined(OS_MACOSX)
   return base::WrapUnique(new UsbServiceImpl(blocking_task_runner));
 #else
   return nullptr;
 #endif
 }
 
 UsbService::~UsbService() {
 #if DCHECK_IS_ON()
   DCHECK(did_shutdown_);
 #endif
   for (const auto& map_entry : devices_)
     map_entry.second->OnDisconnect();
   for (auto& observer : observer_list_)
     observer.WillDestroyUsbService();
 }
 
 UsbService::UsbService(
-    scoped_refptr<base::SingleThreadTaskRunner> task_runner,
     scoped_refptr<base::SequencedTaskRunner> blocking_task_runner)
-    : task_runner_(task_runner), blocking_task_runner_(blocking_task_runner) {}
+    : blocking_task_runner_(std::move(blocking_task_runner)) {
+  if (base::ThreadTaskRunnerHandle::IsSet())
+    task_runner_ = base::ThreadTaskRunnerHandle::Get();
+}
 
 scoped_refptr<UsbDevice> UsbService::GetDevice(const std::string& guid) {
   DCHECK(CalledOnValidThread());
   auto it = devices_.find(guid);
   if (it == devices_.end())
     return nullptr;
   return it->second;
 }
 
 void UsbService::Shutdown() {
@@ skipping 69 matching lines @@
   DCHECK(CalledOnValidThread());
 
   for (auto& observer : observer_list_)
     observer.OnDeviceRemoved(device);
   device->NotifyDeviceRemoved();
   for (auto& observer : observer_list_)
     observer.OnDeviceRemovedCleanup(device);
 }
 
 }  // namespace device