Adding GetEventSource() method to the WindowTreeHost class.

ui/aura/window_tree_host_win.cc

Index: ui/aura/window_tree_host_win.cc
diff --git a/ui/aura/window_tree_host_win.cc b/ui/aura/window_tree_host_win.cc
index 5f1dc00a69127f2bdfd40dd0f30dd649a9d4b3eb..9bbb44b0f535ae02c2f390fc7774f5b5ed85b1e2 100644
--- a/ui/aura/window_tree_host_win.cc
+++ b/ui/aura/window_tree_host_win.cc
@@ -55,6 +55,10 @@ WindowTreeHostWin::~WindowTreeHostWin() {
   DestroyWindow(hwnd());
 }
 
+ui::EventSource* WindowTreeHostWin::GetEventSource() {
+  return this;
+}
+
 gfx::AcceleratedWidget WindowTreeHostWin::GetAcceleratedWidget() {
   return hwnd();
 }