mus: Add views_mus_interactive_ui_tests target

ui/views/test/platform_test_helper.h

 // 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.
 
 #ifndef UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_
 #define UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_
 
 #include <memory>
 
 #include "base/macros.h"
 
 namespace views {
 
 class PlatformTestHelper {
  public:
   PlatformTestHelper() {}
   virtual ~PlatformTestHelper() {}
 
   static std::unique_ptr<PlatformTestHelper> Create();
 
-  virtual bool IsMus() const = 0;
+  // Static so it can be called before Create().
+  static bool IsMus();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PlatformTestHelper);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_