aura-mus: create an interactive ui test for drag and drop.

ui/aura/test/env_test_helper.h

 // 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.
 
 #ifndef UI_AURA_TEST_ENV_TEST_HELPER_H_
 #define UI_AURA_TEST_ENV_TEST_HELPER_H_
 
 #include <utility>
 
 #include "base/macros.h"
@@ skipping 12 matching lines @@
   void SetInputStateLookup(
       std::unique_ptr<InputStateLookup> input_state_lookup) {
     env_->input_state_lookup_ = std::move(input_state_lookup);
   }
 
   void ResetEventState() {
     env_->mouse_button_flags_ = 0;
     env_->is_touch_down_ = false;
   }
 
-  void SetMode(Env::Mode mode) { env_->mode_ = mode; }
+  void SetMode(Env::Mode mode) {
+    env_->mode_ = mode;
+    if (mode == Env::Mode::MUS)
+      env_->EnableMusOSExchangeDataProvider();
+  }
 
   WindowTreeClient* GetWindowTreeClient() { return env_->window_tree_client_; }
 
   // This circumvents the DCHECKs in Env::SetWindowTreeClient() and should
   // only be used for tests where Env is long lived.
   void SetWindowTreeClient(WindowTreeClient* window_tree_client) {
     env_->window_tree_client_ = window_tree_client;
   }
 
   void SetAlwaysUseLastMouseLocation(bool value) {
     env_->always_use_last_mouse_location_ = value;
   }
 
  private:
   Env* env_;
 
   DISALLOW_COPY_AND_ASSIGN(EnvTestHelper);
 };
 
 }  // namespace test
 }  // namespace aura
 
 #endif  // UI_AURA_TEST_ENV_TEST_HELPER_H_