Introduce user customization of external HighDPI mode for 4K monitor.

ash/display/display_manager.h

Index: ash/display/display_manager.h
diff --git a/ash/display/display_manager.h b/ash/display/display_manager.h
index cb1850722ea187170ce1e49ebef318f509a5737b..bcdfe5bbd278c50786fb56e36b6423981219351d 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -82,6 +82,10 @@ class ASH_EXPORT DisplayManager
   // Returns the list of possible UI scales for the display.
   static std::vector<float> GetScalesForDisplay(const DisplayInfo& info);
 
+  // Returns the list of possible device scale factors for the external display.
+  static std::vector<float> GetDeviceScaleFactorsForDisplay(
+      const DisplayInfo& info);
+
   // Returns next valid UI scale.
   static float GetNextUIScale(const DisplayInfo& info, bool up);
 
@@ -167,6 +171,9 @@ class ASH_EXPORT DisplayManager
   // Sets the display's resolution.
   void SetDisplayResolution(int64 display_id, const gfx::Size& resolution);
 
+  // Sets the external display's device scale factor.
+  void SetDisplayDeviceScaleFactor(int64 display_id, float device_scale_factor);
+
   // Register per display properties. |overscan_insets| is NULL if
   // the display has no custom overscan insets.
   void RegisterDisplayProperty(int64 display_id,