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

ui/views/linux_ui/linux_ui.h

Index: ui/views/linux_ui/linux_ui.h
diff --git a/ui/views/linux_ui/linux_ui.h b/ui/views/linux_ui/linux_ui.h
index 088de922431d345fe354ba927850cb942e5bed42..fa910d23f77a01da98fe20300b41aac73def3225 100644
--- a/ui/views/linux_ui/linux_ui.h
+++ b/ui/views/linux_ui/linux_ui.h
@@ -154,6 +154,9 @@ class VIEWS_EXPORT LinuxUI : public ui::LinuxInputMethodContextFactory,
   // Updates the device scale factor so that the default font size can be
   // recalculated.
   virtual void UpdateDeviceScaleFactor(float device_scale_factor) = 0;

[sadrul, 2015/04/09 16:50:01]

Given that LinuxUI is now responsible for getting the DSF (which is used to set
the DSF on the display), do we still need UpdateDeviceScaleFactor()?

[stapelberg, 2015/04/09 18:11:29]

Yes. If I try to call GetDeviceScaleFactor() directly in UpdateDefaultFont(), I
get a segfault because the supported scale factors have not yet been initialized
(we’ve had the same thing in another CL a while ago):

#0  0x0000555556896e27 in ui::GetSupportedScaleFactor(float) ()
#1  0x0000555558fc3e37 in libgtk2ui::Gtk2UI::GetDeviceScaleFactor() const ()
#2  0x0000555558fc2ee8 in
libgtk2ui::Gtk2UI::UpdateDefaultFont(_PangoFontDescription const*) ()
#3  0x0000555558fc09a5 in libgtk2ui::Gtk2UI::LoadGtkValues() ()
#4  0x0000555558fc073d in libgtk2ui::Gtk2UI::Initialize() ()
#5  0x00005555581b0860 in ChromeBrowserMainExtraPartsAura::ToolkitInitialized()
()
#6  0x0000555556283efa in ChromeBrowserMainParts::ToolkitInitialized() ()
#7  0x00005555586a30de in content::BrowserMainLoop::InitializeToolkit() ()
#8  0x00005555584b9760 in
content::BrowserMainRunnerImpl::Initialize(content::MainFunctionParams const&)
()
#9  0x00005555584b947d in content::BrowserMain(content::MainFunctionParams
const&) ()
#10 0x00005555565053d5 in content::ContentMainRunnerImpl::Run() ()
#11 0x0000555556504170 in content::ContentMain(content::ContentMainParams
const&) ()
#12 0x0000555555ffe9f8 in ChromeMain ()
#13 0x00007ffff1523fe0 in __libc_start_main () at /lib64/libc.so.6
#14 0x0000555555ffe8af in _start ()

+
+  // Determines the device scale factor of the primary screen.
+  virtual float GetDeviceScaleFactor() const = 0;
 };
 
 }  // namespace views