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

ui/aura/env.h

Index: ui/aura/env.h
diff --git a/ui/aura/env.h b/ui/aura/env.h
index b9b4d77005e6289fc7de07859d08307372084ebe..1f1ba63c1db34591ac2650dc26ed5cb74166c94e 100644
--- a/ui/aura/env.h
+++ b/ui/aura/env.h
@@ -124,6 +124,11 @@ class AURA_EXPORT Env : public ui::EventTarget,
 
   void Init();
 
+  // After calling this method, all OSExchangeDataProvider instances will be
+  // Mus instances. We can't do this work in Init(), because our mode may
+  // changed via the EnvTestHelper.
+  void EnableMusOSExchangeDataProvider();
+
   // Called by the Window when it is initialized. Notifies observers.
   void NotifyWindowInitialized(Window* window);
 
@@ -162,6 +167,8 @@ class AURA_EXPORT Env : public ui::EventTarget,
   // This may be set to true in tests to force using |last_mouse_location_|
   // rather than querying WindowTreeClient.
   bool always_use_last_mouse_location_ = false;
+  // Whether we set ourselves as the OSExchangeDataProviderFactory.
+  bool is_os_exchange_data_provider_factory_ = false;
 
   std::unique_ptr<InputStateLookup> input_state_lookup_;
   std::unique_ptr<ui::PlatformEventSource> event_source_;