Flip the flag to enable smooth screen rotation by default.

ash/test/ash_test_helper.h

Index: ash/test/ash_test_helper.h
diff --git a/ash/test/ash_test_helper.h b/ash/test/ash_test_helper.h
index a0f7aaf557c92a2a54573270297a9988f8871d8d..a53021f2375fada6b60f902f6fa0e0eaa22099a9 100644
--- a/ash/test/ash_test_helper.h
+++ b/ash/test/ash_test_helper.h
@@ -14,6 +14,7 @@
 #include "ash/test/test_session_controller_client.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/test/scoped_command_line.h"
 #include "ui/aura/test/mus/test_window_tree_client_setup.h"
 
 namespace aura {
@@ -117,6 +118,8 @@ class AshTestHelper {
     session_controller_client_ = std::move(session_controller_client);
   }
 
+  void reset_commandline() { command_line_.reset(); }
+
  private:
   // These TestSuites need to manipulate |config_|.
   friend class AshTestSuite;
@@ -168,6 +171,8 @@ class AshTestHelper {
 
   std::unique_ptr<ui::InputDeviceClient> input_device_client_;
 
+  std::unique_ptr<base::test::ScopedCommandLine> command_line_;
+
   DISALLOW_COPY_AND_ASSIGN(AshTestHelper);
 };