Add API to get top bar height on macOS

atom/browser/api/atom_api_screen_mac.mm

@@ -0,0 +1,15 @@
+#import "atom/browser/api/atom_api_screen.h"
+
+#import <Cocoa/Cocoa.h>
+
+namespace atom {
+
+namespace api {
+#if defined(OS_MACOSX)
+int Screen::getMenuBarHeight(){
+  return [[NSApp mainMenu] menuBarHeight];
+}
+#endif
+}// namespace api
+
+}// namespace atom