Call libusb_init on the FILE thread.

device/usb/usb_context.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 DEVICE_USB_USB_CONTEXT_H_
 #define DEVICE_USB_USB_CONTEXT_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/thread_checker.h"
 
 struct libusb_context;
 
 namespace device {
 
 typedef libusb_context* PlatformUsbContext;
 
 // Ref-counted wrapper for libusb_context*.
 // It also manages the life-cycle of UsbEventHandler.
 // It is a blocking operation to delete UsbContext.
 // Destructor must be called on FILE thread.
 class UsbContext : public base::RefCountedThreadSafe<UsbContext> {
  public:
+  explicit UsbContext(PlatformUsbContext context);
+
   PlatformUsbContext context() const { return context_; }
 
  protected:
-  friend class UsbServiceImpl;
   friend class base::RefCountedThreadSafe<UsbContext>;
 
-  explicit UsbContext(PlatformUsbContext context);
   virtual ~UsbContext();
 
  private:
   class UsbEventHandler;
 
   PlatformUsbContext context_;
   std::unique_ptr<UsbEventHandler> event_handler_;
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(UsbContext);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_CONTEXT_H_