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

components/exo/pointer.cc

Index: components/exo/pointer.cc
diff --git a/components/exo/pointer.cc b/components/exo/pointer.cc
index dd79ff23491f43584f2ec2322e10a17960876b97..0e5e9d1bfff77c2d5c0ffbb2e7bcbe7b4ef0886b 100644
--- a/components/exo/pointer.cc
+++ b/components/exo/pointer.cc
@@ -123,6 +123,10 @@ void Pointer::SetCursor(Surface* surface, const gfx::Point& hotspot) {
         focus_->window()->GetCursor(gfx::ToFlooredPoint(location_)));
 }
 
+bool Pointer::HasStylusDelegate() const {
+  return !!stylus_delegate_;
+}
+
 void Pointer::SetStylusDelegate(PointerStylusDelegate* delegate) {
   stylus_delegate_ = delegate;
 }