Adding GetEventSource() method to the WindowTreeHost class.

mojo/examples/aura_demo/window_tree_host_mojo.cc

Index: mojo/examples/aura_demo/window_tree_host_mojo.cc
diff --git a/mojo/examples/aura_demo/window_tree_host_mojo.cc b/mojo/examples/aura_demo/window_tree_host_mojo.cc
index 7a5bd4e02d9bd420cdd5c7077844e37fd1cb7652..5d935cd259fc9d71c0842bb72696b8148120efd0 100644
--- a/mojo/examples/aura_demo/window_tree_host_mojo.cc
+++ b/mojo/examples/aura_demo/window_tree_host_mojo.cc
@@ -56,6 +56,10 @@ WindowTreeHostMojo::~WindowTreeHostMojo() {}
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHostMojo, aura::WindowTreeHost implementation:
 
+ui::EventSource* WindowTreeHostMojo::GetEventSource() {
+  return this;
+}
+
 gfx::AcceleratedWidget WindowTreeHostMojo::GetAcceleratedWidget() {
   NOTIMPLEMENTED() << "GetAcceleratedWidget";
   return gfx::kNullAcceleratedWidget;