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

ui/views/test/event_generator_delegate_mac.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Cocoa/Cocoa.h>
 #include <stddef.h>
 
 #import "base/mac/scoped_nsobject.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
 #include "base/macros.h"
@@ skipping 364 matching lines @@
   EmulateSendEvent(window_, ns_event);
 }
 
 void EventGeneratorDelegateMac::OnTouchEvent(ui::TouchEvent* event) {
   NOTREACHED() << "Touchscreen events not supported on Chrome Mac.";
 }
 
 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();
   owner_ = owner;
 
   // Retain the NSWindow (note it can be nil). This matches Cocoa's tendency to
   // have autoreleased objects, or objects still in the event queue, that
   // reference the NSWindow.
   window_.reset([window retain]);
 
@@ skipping 89 matching lines @@
 - (NSEvent*)currentEvent {
   if (g_current_event)
     return g_current_event;
 
   // Find the original implementation and invoke it.
   IMP original = EventGeneratorDelegateMac::GetInstance()->CurrentEventMethod();
   return original(self, _cmd);
 }
 
 @end