exo: Produce an error if stylus delegate is already associated.

components/exo/pointer.h

 // Copyright 2015 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 COMPONENTS_EXO_POINTER_H_
 #define COMPONENTS_EXO_POINTER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 28 matching lines @@
   explicit Pointer(PointerDelegate* delegate);
   ~Pointer() override;
 
   // Set the pointer surface, i.e., the surface that contains the pointer image
   // (cursor). The |hotspot| argument defines the position of the pointer
   // surface relative to the pointer location. Its top-left corner is always at
   // (x, y) - (hotspot.x, hotspot.y), where (x, y) are the coordinates of the
   // pointer location, in surface local coordinates.
   void SetCursor(Surface* surface, const gfx::Point& hotspot);
 
+  bool HasStylusDelegate() const;
   // Set delegate for stylus events.
   void SetStylusDelegate(PointerStylusDelegate* delegate);
 
   // Overridden from ui::EventHandler:
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnScrollEvent(ui::ScrollEvent* event) override;
 
   // Overridden from WMHelper::CursorObserver:
   void OnCursorSetChanged(ui::CursorSetType cursor_set) override;
 
@@ skipping 49 matching lines @@
 
   // True if the pointer is controlled via direct input.
   bool is_direct_input_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(Pointer);
 };
 
 }  // namespace exo
 
 #endif  // COMPONENTS_EXO_POINTER_H_