Support NSFlagsChanged in ui::EventFromNative.

ui/events/test/cocoa_test_event_utils.mm

Index: ui/events/test/cocoa_test_event_utils.mm
diff --git a/ui/events/test/cocoa_test_event_utils.mm b/ui/events/test/cocoa_test_event_utils.mm
index c62c482f42bbb85a5330168ffff3b22cc2955a29..8bf1837edb4506a4e8ccec873494be411f5a5382 100644
--- a/ui/events/test/cocoa_test_event_utils.mm
+++ b/ui/events/test/cocoa_test_event_utils.mm
@@ -235,6 +235,20 @@ NSEvent* KeyEventWithKeyCode(unsigned short key_code,
                            keyCode:key_code];
 }
 
+NSEvent* KeyEventWithModifierOnly(unsigned short key_code,
+                                  NSUInteger modifiers) {
+  return [NSEvent keyEventWithType:NSFlagsChanged
+                          location:NSZeroPoint
+                     modifierFlags:modifiers
+                         timestamp:TimeIntervalSinceSystemStartup()
+                      windowNumber:0
+                           context:nil
+                        characters:@""
+       charactersIgnoringModifiers:@""
+                         isARepeat:NO
+                           keyCode:key_code];
+}
+
 static NSEvent* EnterExitEventWithType(NSEventType event_type) {
   return [NSEvent enterExitEventWithType:event_type
                                 location:NSZeroPoint