MacViews: Fix crash when EventMonitorMac receives a NSSystemDefinedEvent

ui/events/cocoa/events_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.
 
 #include "ui/events/event_utils.h"
 
 #include <Cocoa/Cocoa.h>
 
 #include "base/logging.h"
 #import "base/mac/mac_util.h"
@@ skipping 34 matching lines @@
     case NSMouseMoved:
       return ET_MOUSE_MOVED;
     case NSScrollWheel:
       return ET_MOUSEWHEEL;
     case NSMouseEntered:
       return ET_MOUSE_ENTERED;
     case NSMouseExited:
       return ET_MOUSE_EXITED;
     case NSEventTypeSwipe:
       return ET_SCROLL_FLING_START;
-    case NSFlagsChanged:
     case NSAppKitDefined:
     case NSSystemDefined:
+      return ET_UNKNOWN;
+    case NSFlagsChanged:
     case NSApplicationDefined:
     case NSPeriodic:
     case NSCursorUpdate:
     case NSTabletPoint:
     case NSTabletProximity:
     case NSEventTypeGesture:
     case NSEventTypeMagnify:
     case NSEventTypeRotate:
     case NSEventTypeBeginGesture:
     case NSEventTypeEndGesture:
@@ skipping 193 matching lines @@
   uint16 return_value;
   [text getCharacters:&return_value];
   return return_value;
 }
 
 bool IsCharFromNative(const base::NativeEvent& native_event) {
   return false;
 }
 
 }  // namespace ui