Add a feature flag for the new Windows USB backend.

device/usb/usb_service.cc

Index: device/usb/usb_service.cc
diff --git a/device/usb/usb_service.cc b/device/usb/usb_service.cc
index 223a5239ecaa57ac5a017043376699312370a622..80873c0e6e94b3160f968a987510e36e6b238370 100644
--- a/device/usb/usb_service.cc
+++ b/device/usb/usb_service.cc
@@ -5,11 +5,13 @@
 #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)
@@ -17,6 +19,9 @@
 #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
 
@@ -43,7 +48,12 @@ std::unique_ptr<UsbService> UsbService::Create(
   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;
@@ -61,9 +71,11 @@ UsbService::~UsbService() {
 }
 
 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());