Implements a display scale tray setting behind a chrome switch

ui/display/display.h

Index: ui/display/display.h
diff --git a/ui/display/display.h b/ui/display/display.h
index 0e704a4881f4a35fe77f73fe7640acf4906db218..03c9ddd9b8a833f2ef51b3f2752450b77b53e8bb 100644
--- a/ui/display/display.h
+++ b/ui/display/display.h
@@ -86,6 +86,9 @@ class DISPLAY_EXPORT Display final {
   // ensures that the command line is reevaluated.
   static void ResetForceDeviceScaleFactorForTesting();
 
+  // Resets the cache and sets a new force device scale factor.
+  static void SetForceDeviceScaleFactor(double dsf);
+
   // Sets/Gets unique identifier associated with the display.
   // -1 means invalid display and it doesn't not exit.
   int64_t id() const { return id_; }