Adding GetEventSource() method to the WindowTreeHost class.

ui/aura/window_tree_host_x11.cc

Index: ui/aura/window_tree_host_x11.cc
diff --git a/ui/aura/window_tree_host_x11.cc b/ui/aura/window_tree_host_x11.cc
index 22b0bfeed28bed3758a2e5c9dbe0f89704026924..fe2489488689aeaaeac96ca97525b1daa04a1fb5 100644
--- a/ui/aura/window_tree_host_x11.cc
+++ b/ui/aura/window_tree_host_x11.cc
@@ -342,6 +342,10 @@ uint32_t WindowTreeHostX11::DispatchEvent(const ui::PlatformEvent& event) {
   return ui::POST_DISPATCH_STOP_PROPAGATION;
 }
 
+ui::EventSource* WindowTreeHostX11::GetEventSource() {
+  return this;
+}
+
 gfx::AcceleratedWidget WindowTreeHostX11::GetAcceleratedWidget() {
   return xwindow_;
 }