Reland: mus: Add views_mus_interactive_ui_tests target

ui/views/test/platform_test_helper.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/views/test/platform_test_helper.h"
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 
 namespace views {
 namespace {
 
 class DefaultPlatformTestHelper : public PlatformTestHelper {
  public:
   DefaultPlatformTestHelper() {}
 
   ~DefaultPlatformTestHelper() override {}
 
-  bool IsMus() const override { return false; }
-
  private:
   DISALLOW_COPY_AND_ASSIGN(DefaultPlatformTestHelper);
 };
 
 PlatformTestHelper::Factory test_helper_factory;
+bool is_mus = false;
 
 }  // namespace
 
 void PlatformTestHelper::set_factory(const Factory& factory) {
   DCHECK(test_helper_factory.is_null());
   test_helper_factory = factory;
 }
 
 // static
 std::unique_ptr<PlatformTestHelper> PlatformTestHelper::Create() {
   return !test_helper_factory.is_null()
              ? test_helper_factory.Run()
              : base::WrapUnique(new DefaultPlatformTestHelper);
 }
 
+// static
+void PlatformTestHelper::SetIsMus() {
+  is_mus = true;
+}
+
+// static
+bool PlatformTestHelper::IsMus() {
+  return is_mus;
+}
+
 }  // namespace views