Adding GetEventSource() method to the WindowTreeHost class.

ui/aura/window_tree_host_mac.mm

Index: ui/aura/window_tree_host_mac.mm
diff --git a/ui/aura/window_tree_host_mac.mm b/ui/aura/window_tree_host_mac.mm
index dbfa8bbcc79adcb78ef4b76989e4442dcdb40bfe..020aabde114539dbaf38ba04d730a108840a8365 100644
--- a/ui/aura/window_tree_host_mac.mm
+++ b/ui/aura/window_tree_host_mac.mm
@@ -23,6 +23,11 @@ WindowTreeHostMac::~WindowTreeHostMac() {
   DestroyDispatcher();
 }
 
+EventSource* WindowTreeHostMac::GetEventSource() {
+  NOTIMPLEMENTED();
+  return nil;
+}
+
 gfx::AcceleratedWidget WindowTreeHostMac::GetAcceleratedWidget() {
   return [window_ contentView];
 }