Get the default device scale factor from monitor

ui/gfx/monitor.h

Index: ui/gfx/monitor.h
diff --git a/ui/gfx/monitor.h b/ui/gfx/monitor.h
index f9bf86c62ddee1ee5d47d5cc3f82992954531ee2..704d6c2e55f291f0190b5161117dc328e7c6fc41 100644
--- a/ui/gfx/monitor.h
+++ b/ui/gfx/monitor.h
@@ -20,6 +20,10 @@ namespace gfx {
 // TODO(oshima): Update the comment when ENABLE_DIP macro is removed.
 class UI_EXPORT Monitor {
  public:
+  // Returns the default value for the device scale factor, which is
+  // given by "--default-device-scale-factor".
+  static float GetDefaultDeviceScaleFactor();
+
   // Creates a monitor with invalid id(-1) as default.
   Monitor();
   explicit Monitor(int id);