[Ozone] Fix DCHECK and check for scroll events as well

Scroll events are mouse events as well. On debug builds this DCHECK
would otherwise get triggered.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/590523004

Cr-Commit-Position: refs/heads/master@{#295772}

ui/events/ozone/events_ozone.cc

@@ -44,7 +44,7 @@ int GetChangedMouseButtonFlagsFromNative(
     const base::NativeEvent& native_event) {
   const ui::MouseEvent* event =
       static_cast<const ui::MouseEvent*>(native_event);
-  DCHECK(event->IsMouseEvent());
+  DCHECK(event->IsMouseEvent() || event->IsScrollEvent());
   return event->changed_button_flags();
 }