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

components/exo/pointer.cc

 // 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.
 
 #include "components/exo/pointer.h"
 
 #include "ash/public/cpp/shell_window_ids.h"
 #include "components/exo/pointer_delegate.h"
 #include "components/exo/pointer_stylus_delegate.h"
 #include "components/exo/surface.h"
@@ skipping 105 matching lines @@
 
   // Update cursor in case the registration of pointer as cursor provider
   // caused the cursor to change.
   aura::client::CursorClient* cursor_client =
       aura::client::GetCursorClient(focus_->window()->GetRootWindow());
   if (cursor_client)
     cursor_client->SetCursor(
         focus_->window()->GetCursor(gfx::ToFlooredPoint(location_)));
 }
 
+bool Pointer::HasStylusDelegate() const {
+  return !!stylus_delegate_;
+}
+
 void Pointer::SetStylusDelegate(PointerStylusDelegate* delegate) {
   stylus_delegate_ = delegate;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // ui::EventHandler overrides:
 
 void Pointer::OnMouseEvent(ui::MouseEvent* event) {
   Surface* target = GetEffectiveTargetForEvent(event);
 
@@ skipping 233 matching lines @@
 
   if (ui_scale != cursor_scale_) {
     gfx::Transform transform;
     transform.Scale(ui_scale, ui_scale);
     widget_->GetNativeWindow()->SetTransform(transform);
     cursor_scale_ = ui_scale;
   }
 }
 
 }  // namespace exo