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

ash/display/display_info.h

Index: ash/display/display_info.h
diff --git a/ash/display/display_info.h b/ash/display/display_info.h
index a8bca41e4d7bf7371034a1183728d51378016572..c544956fd4e717709df6ac9d0baec11f1ca0a819 100644
--- a/ash/display/display_info.h
+++ b/ash/display/display_info.h
@@ -24,10 +24,16 @@ struct ASH_EXPORT DisplayMode {
               bool interlaced,
               bool native);
 
+  // Returns the size in DIP which isvisible to the user.
+  gfx::Size GetSizeInDIP() const;
+
   gfx::Size size;      // Physical pixel size of the display.
   float refresh_rate;  // Refresh rate of the display, in Hz.
   bool interlaced;     // True if mode is interlaced.
   bool native;         // True if mode is native mode of the display.
+  float ui_scale;      // The UI scale factor of the mode.
+                       // TODO(mukai|oshima): merge to device_scale_factor.
+  float device_scale_factor;  // The device scale factor of the mode.
 };
 
 // DisplayInfo contains metadata for each display. This is used to