Remove now-unnecessary wrappers around gfx::Screen::GetScreen()

extensions/browser/api/system_display/system_display_apitest.cc

 // 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.
 
 #include <stdint.h>
 
 #include <utility>
 
 #include "base/debug/leak_annotations.h"
 #include "base/macros.h"
@@ skipping 70 matching lines @@
   bool SetInfo(const std::string& display_id,
                const api::system_display::DisplayProperties& params,
                std::string* error) override {
     // Should get called only once per test case.
     EXPECT_FALSE(set_info_value_);
     set_info_value_ = params.ToValue();
     set_info_display_id_ = display_id;
     return true;
   }
 
-  gfx::Screen* GetActiveScreen() override { return NULL; }
-
   void EnableUnifiedDesktop(bool enable) override {
     unified_desktop_enabled_ = enable;
   }
 
   scoped_ptr<base::DictionaryValue> GetSetInfoValue() {
     return std::move(set_info_value_);
   }
 
   std::string GetSetInfoDisplayId() const { return set_info_display_id_; }
 
@@ skipping 192 matching lines @@
     enable_unified_function->set_extension(test_extension.get());
     ASSERT_TRUE(api_test_utils::RunFunction(enable_unified_function.get(),
                                             "[false]", browser_context()));
     EXPECT_FALSE(provider_->unified_desktop_enabled());
   }
 }
 
 #endif  // defined(OS_CHROMEOS)
 
 }  // namespace extensions