Disable screen rotation when splitview is engaged and only allow splitview in landscape.

athena/wm/split_view_controller.h

Index: athena/wm/split_view_controller.h
diff --git a/athena/wm/split_view_controller.h b/athena/wm/split_view_controller.h
index 56c33c720ff3c954869f9f68d88d44795c25344f..c6c5e8e23c01f35624982a1a85fe7c0ca24bc7ef 100644
--- a/athena/wm/split_view_controller.h
+++ b/athena/wm/split_view_controller.h
@@ -17,6 +17,7 @@ class Transform;
 
 namespace athena {
 class WindowListProvider;
+class SplitViewControllerTest;
 
 // Responsible for entering split view mode, exiting from split view mode, and
 // laying out the windows in split view mode.
@@ -55,6 +56,8 @@ class ATHENA_EXPORT SplitViewController
   aura::Window* right_window() { return right_window_; }
 
  private:
+  friend class SplitViewControllerTest;
+
   enum State {
     // Split View mode is not active. |left_window_| and |right_window| are
     // NULL.
@@ -68,6 +71,7 @@ class ATHENA_EXPORT SplitViewController
     ACTIVE
   };
 
+  void SetState(State state);
   void UpdateLayout(bool animate);
 
   void SetWindowTransforms(const gfx::Transform& left_transform,