Allow OSExchangeData::Provider to be overridden at run time.

ui/base/dragdrop/os_exchange_data_provider_builder_mac.mm

+// Copyright 2016 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.
+
+#include "ui/base/dragdrop/os_exchange_data_provider_builder_mac.h"
+
+#include "base/memory/ptr_util.h"
+#include "ui/base/dragdrop/os_exchange_data_provider_mac.h"
+
+namespace ui {
+
+std::unique_ptr<OSExchangeData::Provider> BuildOSExchangeDataProviderMac() {
+  return base::MakeUnique<OSExchangeDataProviderMac>();
+}
+
+}  // namespace ui