Adding GetEventSource() method to the WindowTreeHost class.

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
index efd24b6b76c207d9bc331f412f99a784cd8bafe2..386aa8b93934d7958a6ca055acb24cae4879f2b8 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
@@ -765,6 +765,10 @@ bool DesktopWindowTreeHostX11::IsAnimatingClosed() const {
 ////////////////////////////////////////////////////////////////////////////////
 // DesktopWindowTreeHostX11, aura::WindowTreeHost implementation:
 
+ui::EventSource* DesktopWindowTreeHostX11::GetEventSource() {
+  return this;
+}
+
 gfx::AcceleratedWidget DesktopWindowTreeHostX11::GetAcceleratedWidget() {
   return xwindow_;
 }