MacViews: Fix crash when EventMonitorMac receives a system defined event.

EventFromNative() returns null when given a system defined event, so filter them
out when monitoring the event stream.
When running the experimental MacViews browser, I was seeing an event
with type=SysDefined subtype=7 (apparently it means that a mouse button
state has changed).

BUG=425229

Review URL: https://codereview.chromium.org/981693002

Cr-Commit-Position: refs/heads/master@{#319142}

ui/events/cocoa/events_mac.mm

@@ -52,9 +52,10 @@ EventType EventTypeFromNative(const base::NativeEvent& native_event) {
       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:

ui/views/event_monitor_mac.mm

@@ -42,7 +42,8 @@ EventMonitorMac::EventMonitorMac(ui::EventHandler* event_handler,
       handler:^NSEvent*(NSEvent* event) {
           if (!target_window || [event window] == target_window) {
             scoped_ptr<ui::Event> ui_event = ui::EventFromNative(event);
-            event_handler->OnEvent(ui_event.get());
+            if (ui_event)
+              event_handler->OnEvent(ui_event.get());
           }
           return event;
       }];