Implements a display scale tray setting behind a chrome switch

ui/display/display.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_DISPLAY_DISPLAY_H_
 #define UI_DISPLAY_DISPLAY_H_
 
 #include <stdint.h>
 
 #include "base/compiler_specific.h"
@@ skipping 68 matching lines @@
 
   // Indicates if a device scale factor is being explicitly enforced from the
   // command line via "--force-device-scale-factor".
   static bool HasForceDeviceScaleFactor();
 
   // Resets the caches used to determine if a device scale factor is being
   // forced from the command line via "--force-device-scale-factor", and thus
   // 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_; }
   void set_id(int64_t id) { id_ = id; }
 
   // Gets/Sets the display's bounds in Screen's coordinates.
   const gfx::Rect& bounds() const { return bounds_; }
   void set_bounds(const gfx::Rect& bounds) { bounds_ = bounds; }
 
   // Gets/Sets the display's work area in Screen's coordinates.
@@ skipping 109 matching lines @@
   gfx::Size maximum_cursor_size_;
   gfx::ICCProfile icc_profile_;
   int color_depth_;
   int depth_per_component_;
   bool is_monochrome_ = false;
 };
 
 }  // namespace display
 
 #endif  // UI_DISPLAY_DISPLAY_H_