Support NSFlagsChanged in ui::EventFromNative.

ui/events/test/cocoa_test_event_utils.h

Index: ui/events/test/cocoa_test_event_utils.h
diff --git a/ui/events/test/cocoa_test_event_utils.h b/ui/events/test/cocoa_test_event_utils.h
index dbb538f570e98449f786c430a36127969d326aff..7267693c027abc3ef2d49dd0bd600b94887c238d 100644
--- a/ui/events/test/cocoa_test_event_utils.h
+++ b/ui/events/test/cocoa_test_event_utils.h
@@ -74,6 +74,13 @@ NSEvent* KeyEventWithKeyCode(unsigned short key_code,
                              NSEventType event_type,
                              NSUInteger modifiers);
 
+// Returns a key event for pressing or releasing a modifier key (aka
+// NSFlagsChanged). For example |key_code| == kVK_Shift with |modifiers| &
+// NSShiftKeyMask == true means Shift is pressed and |key_code| == kVK_Shift

[tapted, 2016/10/25 06:55:10]

nit: `|modifiers| & NSShiftKeyMask == true` -> `(|modifiers| & NSShiftKeyMask)
!= 0`

[alshabalin, 2016/10/28 19:58:14]

Done.

+// with |modifiers| & NSShiftKeyMask == false means Shift is released.

[tapted, 2016/10/25 06:55:10]

`|modifiers| & NSShiftKeyMask == false` -> `(|modifiers| & NSShiftKeyMask) == 0`

[alshabalin, 2016/10/28 19:58:14]

Done.

+NSEvent* KeyEventWithModifierOnly(unsigned short key_code,
+                                  NSUInteger modifiers);
+
 // Returns a mouse enter event.
 NSEvent* EnterEvent();