Support a ui::test::EventGenerator constructed with a single argument on Mac

ui/views/test/event_generator_delegate_mac.mm

Index: ui/views/test/event_generator_delegate_mac.mm
diff --git a/ui/views/test/event_generator_delegate_mac.mm b/ui/views/test/event_generator_delegate_mac.mm
index a005dbca212fa68b147096c127affe3ae6a50e66..a1ab5cb466b875c339e567b8e9b1ace56adc840d 100644
--- a/ui/views/test/event_generator_delegate_mac.mm
+++ b/ui/views/test/event_generator_delegate_mac.mm
@@ -382,6 +382,11 @@ void EventGeneratorDelegateMac::OnTouchEvent(ui::TouchEvent* event) {
 void EventGeneratorDelegateMac::SetContext(ui::test::EventGenerator* owner,
                                            gfx::NativeWindow root_window,
                                            gfx::NativeWindow window) {
+  // Mac doesn't use a |root_window|. Assume that if a single-argument
+  // constructor was used, it should be the actual |window|.
+  if (!window)
+    window = root_window;
+
   swizzle_pressed_.reset();
   swizzle_location_.reset();
   swizzle_current_event_.reset();