Code:   const UsbDeviceHandle::TransferCallback& callback() const {
    return callback_;
   }

Commit Message: Update helper classes in usb_device_handle_unittest for OnceCallback

Helper classes in usb_device_handle_unittest.cc don't fit to OnceCallback
migration, as they are copied and passed to others.
This CL updates them to pass new callbacks for each use to avoid the
copy of callbacks.

Bug: 714018
Change-Id: Ifb70901439ae92b6b049b84534283c39ebc40ee0
Reviewed-on: https://chromium-review.googlesource.com/527549
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478549}
CWE ID: 
  const UsbDeviceHandle::TransferCallback& callback() const {
  UsbDeviceHandle::TransferCallback GetCallback() {
    return base::Bind(&TestCompletionCallback::SetResult,
                      base::Unretained(this));
   }
