Remove BrowserThread dependency from usb_service.

components/usb_service/usb_service.h

 // 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.
 
 #ifndef COMPONENTS_USB_SERVICE_USB_SERVICE_H_
 #define COMPONENTS_USB_SERVICE_USB_SERVICE_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/threading/non_thread_safe.h"
 #include "components/usb_service/usb_service_export.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace usb_service {
 
 class UsbDevice;
 
 // The USB service handles creating and managing an event handler thread that is
 // used to manage and dispatch USB events. It is also responsible for device
 // discovery on the system, which allows it to re-use device handles to prevent
 // competition for the same USB device.
 class USB_SERVICE_EXPORT UsbService : public base::NonThreadSafe {
  public:
-  // Must be called on FILE thread.
-  // Returns NULL when failed to initialized.
-  static UsbService* GetInstance();
+  // Must be called on a thread with a MessageLoopForIO (for example
+  // BrowserThread::FILE). The UI task runner reference is used to talk to the
+  // PermissionBrokerClient on ChromeOS (UI thread). Returns NULL when
+  // initialization fails.
+  static UsbService* GetInstance(
+      scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
 
   static void SetInstanceForTest(UsbService* instance);
 
   virtual scoped_refptr<UsbDevice> GetDeviceById(uint32 unique_id) = 0;
 
   // Get all of the devices attached to the system, inserting them into
   // |devices|. Clears |devices| before use. The result will be sorted by id
   // in increasing order. Must be called on FILE thread.
   virtual void GetDevices(std::vector<scoped_refptr<UsbDevice> >* devices) = 0;
 
  protected:
   friend struct base::DefaultDeleter<UsbService>;
   UsbService() {}
   virtual ~UsbService() {}
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace usb_service
 
 #endif  // COMPONENTS_USB_SERVICE_USB_SERVICE_H_