Allow upgrading to 2x DSF for lower UI scale when 2x resoruces are available.

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 1aa0583ec7330ce9b81ce4fc9d31e84b98667a82..315985379b16c12282da056ea476496d2f454f10 100644
--- a/ash/display/display_info.cc
+++ b/ash/display/display_info.cc
@@ -21,6 +21,11 @@
 #endif
 
 namespace ash {
+namespace {
+
+bool allow_upgrade_to_high_dpi = false;
+
+}
 
 DisplayMode::DisplayMode()
     : refresh_rate(0.0f), interlaced(false), native(false) {}
@@ -40,6 +45,11 @@ DisplayInfo DisplayInfo::CreateFromSpec(const std::string& spec) {
 }
 
 // static
+void DisplayInfo::SetAllowUpgradeToHighDPI(bool enable) {
+  allow_upgrade_to_high_dpi = enable;
+}
+
+// static
 DisplayInfo DisplayInfo::CreateFromSpecWithID(const std::string& spec,
                                               int64 id) {
   // Default bounds for a display.
@@ -235,9 +245,23 @@ void DisplayInfo::SetBounds(const gfx::Rect& new_bounds_in_native) {
   UpdateDisplaySize();
 }
 
+float DisplayInfo::GetEffectiveDeviceScaleFactor() const {
+  if (allow_upgrade_to_high_dpi && configured_ui_scale_ < 1.0f &&
+      device_scale_factor_ == 1.0f) {
+    return 2.0f;
+  } else if (device_scale_factor_ == 2.0f && configured_ui_scale_ == 2.0f) {
+    return 1.0f;
+  }
+  return device_scale_factor_;
+}
+
 float DisplayInfo::GetEffectiveUIScale() const {
-  if (device_scale_factor_ == 2.0f && configured_ui_scale_ == 2.0f)
+  if (allow_upgrade_to_high_dpi && configured_ui_scale_ < 1.0f &&
+      device_scale_factor_ == 1.0f) {
+    return configured_ui_scale_ * 2.0f;
+  } else if (device_scale_factor_ == 2.0f && configured_ui_scale_ == 2.0f) {
     return 1.0f;
+  }
   return configured_ui_scale_;
 }