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

ash/display/display_info.cc

Index: ash/display/display_info.cc
diff --git a/ash/display/display_info.cc b/ash/display/display_info.cc
index 087f0a36a878b49b61310939c28083129c2f8e32..73395dd10ccc345e185bacc84162866b559b5f36 100644
--- a/ash/display/display_info.cc
+++ b/ash/display/display_info.cc
@@ -29,7 +29,11 @@ bool allow_upgrade_to_high_dpi = false;
 }
 
 DisplayMode::DisplayMode()
-    : refresh_rate(0.0f), interlaced(false), native(false) {}
+    : refresh_rate(0.0f),
+      interlaced(false),
+      native(false),
+      ui_scale(1.0f),
+      device_scale_factor(1.0f) {}
 
 DisplayMode::DisplayMode(const gfx::Size& size,
                          float refresh_rate,
@@ -38,7 +42,16 @@ DisplayMode::DisplayMode(const gfx::Size& size,
     : size(size),
       refresh_rate(refresh_rate),
       interlaced(interlaced),
-      native(native) {}
+      native(native),
+      ui_scale(1.0f),
+      device_scale_factor(1.0f) {}
+
+gfx::Size DisplayMode::GetSizeInDIP() const {
+  gfx::SizeF size_dip(size);
+  size_dip.Scale(ui_scale);
+  size_dip.Scale(1.0f / device_scale_factor);
+  return gfx::ToFlooredSize(size_dip);
+}
 
 // satic
 DisplayInfo DisplayInfo::CreateFromSpec(const std::string& spec) {