Fix 'not all switch case are handled' warning in OSWindow.cpp

Change-Id: Ib4d1e4b04c73e5eb5e0e4f6c56212e99723d2ce0
Reviewed-on: https://chromium-review.googlesource.com/269126
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>

util/OSWindow.cpp

@@ -63,6 +63,8 @@ void OSWindow::pushEvent(Event event)
         mWidth = event.Size.Width;
         mHeight = event.Size.Height;
         break;
+      default:
+        break;
     }
 
     mEvents.push_back(event);