device scale factor detection: use gtk-xft-dpi consistently

chrome/browser/ui/libgtk2ui/gtk2_ui.cc

Index: chrome/browser/ui/libgtk2ui/gtk2_ui.cc
diff --git a/chrome/browser/ui/libgtk2ui/gtk2_ui.cc b/chrome/browser/ui/libgtk2ui/gtk2_ui.cc
index 2f7f2cd93cfb7e6629ebaa96c8f8b5012a4cea20..a57b59fb7905762f861446e6587b52f536adf2dc 100644
--- a/chrome/browser/ui/libgtk2ui/gtk2_ui.cc
+++ b/chrome/browser/ui/libgtk2ui/gtk2_ui.cc
@@ -380,16 +380,20 @@ gfx::FontRenderParams GetGtkFontRenderParams() {
   return params;
 }
 
-// Queries GTK for its font DPI setting and returns the number of pixels in a
-// point.
-double GetPixelsInPoint(float device_scale_factor) {
+double GetDPI() {
   GtkSettings* gtk_settings = gtk_settings_get_default();
   CHECK(gtk_settings);
   gint gtk_dpi = -1;
   g_object_get(gtk_settings, "gtk-xft-dpi", &gtk_dpi, NULL);
 
   // GTK multiplies the DPI by 1024 before storing it.
-  double dpi = (gtk_dpi > 0) ? gtk_dpi / 1024.0 : 96.0;
+  return (gtk_dpi > 0) ? gtk_dpi / 1024.0 : 96.0;
+}
+
+// Queries GTK for its font DPI setting and returns the number of pixels in a
+// point.
+double GetPixelsInPoint(float device_scale_factor) {
+  double dpi = GetDPI();
 
   // Take device_scale_factor into account — if Chrome already scales the
   // entire UI up by 2x, we should not also scale up.
@@ -1420,6 +1424,12 @@ void Gtk2UI::UpdateDeviceScaleFactor(float device_scale_factor) {
   UpdateDefaultFont(label_style->font_desc);
 }
 
+float Gtk2UI::GetDeviceScaleFactor() const {
+  const int kCSSDefaultDPI = 96;
+  const float scale = GetDPI() / kCSSDefaultDPI;
+  return ui::GetScaleForScaleFactor(ui::GetSupportedScaleFactor(scale));

[oshima, 2015/04/08 23:57:41]

you may want to add more scales for linux (133 150 etc).

[stapelberg, 2015/04/09 06:49:34]

Yeah, I’ll investigate that in a separate CL so that we can revert them
separately if necessary.

[oshima, 2015/04/09 17:22:23]

I think my comment was a bit confusing. Technically, you can just pass scale,
although we don't want to allow arbitrary scale factor. On Linux, we can pick
something like 1.25, 1.5 and 2.0 and use closest scale factor. (ChromeOS
currently support 1.0, 1.25 and 2.0)

+}
+
 }  // namespace libgtk2ui
 
 views::LinuxUI* BuildGtk2UI() {