Adding GetEventSource() method to the WindowTreeHost class.

ui/aura/window_tree_host.h

Index: ui/aura/window_tree_host.h
diff --git a/ui/aura/window_tree_host.h b/ui/aura/window_tree_host.h
index 55d119e9953818760613dcae9a0b47a80de6b9c1..8e9bbd4cc526fd9981861db65bae61953a272892 100644
--- a/ui/aura/window_tree_host.h
+++ b/ui/aura/window_tree_host.h
@@ -11,6 +11,7 @@
 #include "base/message_loop/message_loop.h"
 #include "ui/aura/aura_export.h"
 #include "ui/base/cursor/cursor.h"
+#include "ui/events/event_source.h"
 #include "ui/gfx/native_widget_types.h"
 
 namespace gfx {
@@ -111,6 +112,10 @@ class AURA_EXPORT WindowTreeHost {
 
   gfx::NativeCursor last_cursor() const { return last_cursor_; }
 
+  // Returns the EventSource responsible for dispatching events to the window
+  // tree.
+  virtual ui::EventSource* GetEventSource() = 0;
+
   // Returns the accelerated widget.
   virtual gfx::AcceleratedWidget GetAcceleratedWidget() = 0;