[views-mus] Prevent creating a native OzonePlatform in mus tests.

ui/aura/mus/os_exchange_data_provider_mus_unittest.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/aura/mus/os_exchange_data_provider_mus.h"
 
 #include <memory>
 
 #include "base/files/file_util.h"
 #include "base/memory/ptr_util.h"
@@ skipping 21 matching lines @@
 
 class OSExchangeDataProviderMusTest
     : public PlatformTest,
       public ui::OSExchangeDataProviderFactory::Factory {
  public:
   OSExchangeDataProviderMusTest() {}
 
   // Overridden from PlatformTest:
   void SetUp() override {
     PlatformTest::SetUp();
+    old_factory_ = ui::OSExchangeDataProviderFactory::TakeFactory();
     ui::OSExchangeDataProviderFactory::SetFactory(this);
   }
 
   void TearDown() override {
-    ui::OSExchangeDataProviderFactory::SetFactory(nullptr);
+    ui::OSExchangeDataProviderFactory::TakeFactory();
+    ui::OSExchangeDataProviderFactory::SetFactory(old_factory_);
     PlatformTest::TearDown();
   }
 
   // Overridden from ui::OSExchangeDataProviderFactory::Factory:
   std::unique_ptr<OSExchangeData::Provider> BuildProvider() override {
     return base::MakeUnique<OSExchangeDataProviderMus>();
   }
 
  private:
+  ui::OSExchangeDataProviderFactory::Factory* old_factory_ = nullptr;
   base::MessageLoopForUI message_loop_;
 };
 
 TEST_F(OSExchangeDataProviderMusTest, StringDataGetAndSet) {
   OSExchangeData data;
   base::string16 input = base::ASCIIToUTF16("I can has cheezburger?");
   EXPECT_FALSE(data.HasString());
   data.SetString(input);
   EXPECT_TRUE(data.HasString());
 
@@ skipping 134 matching lines @@
   data.SetHtml(html, url);
 
   OSExchangeData copy(
       std::unique_ptr<OSExchangeData::Provider>(data.provider().Clone()));
   base::string16 read_html;
   EXPECT_TRUE(copy.GetHtml(&read_html, &url));
   EXPECT_EQ(html, read_html);
 }
 
 }  // namespace aura