Move UsbService to its own component.

chrome/browser/usb/usb_context.h

-// Copyright 2013 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 CHROME_BROWSER_USB_USB_CONTEXT_H_
-#define CHROME_BROWSER_USB_USB_CONTEXT_H_
-
-#include "base/basictypes.h"
-#include "base/memory/ref_counted.h"
-#include "base/memory/scoped_ptr.h"
-#include "base/threading/thread_checker.h"
-
-struct libusb_context;
-
-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:
-  PlatformUsbContext context() const { return context_; }
-
- protected:
-  friend class UsbService;
-  friend class base::RefCountedThreadSafe<UsbContext>;
-
-  explicit UsbContext(PlatformUsbContext context);
-  virtual ~UsbContext();
-
- private:
-  class UsbEventHandler;
-  PlatformUsbContext context_;
-  UsbEventHandler* event_handler_;
-  base::ThreadChecker thread_checker_;
-
-  DISALLOW_COPY_AND_ASSIGN(UsbContext);
-};
-
-#endif  // CHROME_BROWSER_USB_USB_CONTEXT_H_