mus/views: Create a mus OSExchangeData::Provider stub, and use it.

ui/base/dragdrop/os_exchange_data_provider_factory.cc

 // 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_factory.h"
 
 #include "base/memory/ptr_util.h"
 
 #if defined(USE_X11) && !defined(OS_CHROMEOS)
 #include "ui/base/dragdrop/os_exchange_data_provider_aurax11.h"
 #elif defined(OS_LINUX)
 #include "ui/base/dragdrop/os_exchange_data_provider_aura.h"
 #elif defined(OS_MACOSX)
 #include "ui/base/dragdrop/os_exchange_data_provider_builder_mac.h"
 #elif defined(OS_WIN)
 #include "ui/base/dragdrop/os_exchange_data_provider_win.h"
 #endif
 
 namespace ui {
 
 OSExchangeDataProviderFactory::Factory* factory_ = nullptr;
 
 // static
 void OSExchangeDataProviderFactory::SetFactory(Factory* factory) {
+  DCHECK(!factory_ || !factory);
   factory_ = factory;
 }
 
 //static
 std::unique_ptr<OSExchangeData::Provider>
 OSExchangeDataProviderFactory::CreateProvider() {
   if (factory_)
     return factory_->BuildProvider();
 
 #if defined(USE_X11) && !defined(OS_CHROMEOS)
   return base::MakeUnique<OSExchangeDataProviderAuraX11>();
 #elif defined(OS_LINUX)
   return base::MakeUnique<OSExchangeDataProviderAura>();
 #elif defined(OS_MACOSX)
   return ui::BuildOSExchangeDataProviderMac();
 #elif defined(OS_WIN)
   return base::MakeUnique<OSExchangeDataProviderWin>();
 #else
 #error "Unknown operating system"
 #endif
 }
 
 }  // namespace ui