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

ui/views/widget/desktop_aura/desktop_screen_x11.cc

Index: ui/views/widget/desktop_aura/desktop_screen_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_screen_x11.cc b/ui/views/widget/desktop_aura/desktop_screen_x11.cc
index 019688fb9463fb518681202d611a0fc90160a6b2..8673c20af50e65a6446bcce99132759cc5d0aa11 100644
--- a/ui/views/widget/desktop_aura/desktop_screen_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_screen_x11.cc
@@ -295,6 +295,13 @@ std::vector<gfx::Display> DesktopScreenX11::BuildDisplaysFromXRandRInfo() {
   }
 
   float device_scale_factor = 1.0f;
+  {
+    // As per-display scale factor is not supported right now,
+    // the X11 root window's scale factor is always used.
+    // This is what users can change using e.g. xrandr -dpi 96
+    std::vector<gfx::Display> fallback_displays = GetFallbackDisplayList();
+    device_scale_factor = fallback_displays[0].device_scale_factor();
+  }
   for (int i = 0; i < resources->noutput; ++i) {
     RROutput output_id = resources->outputs[i];
     gfx::XScopedPtr<XRROutputInfo,
@@ -321,14 +328,6 @@ std::vector<gfx::Display> DesktopScreenX11::BuildDisplaysFromXRandRInfo() {
       gfx::Display display(display_id, crtc_bounds);
 
       if (!gfx::Display::HasForceDeviceScaleFactor()) {
-        if (i == 0 && !ui::IsDisplaySizeBlackListed(
-            gfx::Size(output_info->mm_width, output_info->mm_height))) {
-          // As per display scale factor is not supported right now,
-          // the primary display's scale factor is always used.
-          device_scale_factor = GetDeviceScaleFactor(crtc->width,
-                                                     output_info->mm_width);
-          DCHECK_LE(1.0f, device_scale_factor);
-        }
         display.SetScaleAndBounds(device_scale_factor, crtc_bounds);
       }