Enable hidpi on Linux, refactor a bit on Windows to share

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 bb253cff17ce48e6295bbee89574a874f5733462..9ee607323ebe12bf6a1a7e0930499f27e0ffff82 100644
--- a/ui/views/widget/desktop_aura/desktop_screen_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_screen_x11.cc
@@ -20,6 +20,7 @@
 #include "ui/display/util/x11/edid_parser_x11.h"
 #include "ui/events/platform/platform_event_source.h"
 #include "ui/gfx/display.h"
+#include "ui/gfx/linux_font_delegate.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/screen.h"
 #include "ui/gfx/x/x11_types.h"
@@ -33,17 +34,31 @@ namespace {
 // in |Dispatch()|.
 const int64 kConfigureDelayMs = 500;
 
-// TODO(oshima): Consider using gtk-xft-dpi instead.
-float GetDeviceScaleFactor(int screen_pixels, int screen_mm) {
-  const int kCSSDefaultDPI = 96;
-  const float kInchInMm = 25.4f;
+double GetDPI() {
+  static double dpi = -1.0;
+  if (dpi < 0.0) {
+    const gfx::LinuxFontDelegate* delegate = gfx::LinuxFontDelegate::instance();
+    if (delegate)
+      dpi = delegate->GetFontDPI();
+    if (dpi <= 0.0)
+      dpi = 96.0;
+  }
+  return dpi;
+}
 
-  float screen_inches = screen_mm / kInchInMm;
-  float screen_dpi = screen_pixels / screen_inches;
-  float scale = screen_dpi / kCSSDefaultDPI;
+float GetGlobalDeviceScaleFactor() {
+  return GetDPI() / 96.0;
+}
 
-  return ui::GetScaleForScaleFactor(ui::GetSupportedScaleFactor(scale));
+// Returns the number of pixels in a point.
+// - multiply a point size by this to get pixels ("device units")
+// - divide a pixel size by this to get points
+#if 0
+double GetPixelsInPoint() {
+  static double pixels_in_point = GetDPI() / 72.0;  // 72 points per inch.
+  return pixels_in_point;
 }
+#endif
 
 std::vector<gfx::Display> GetFallbackDisplayList() {
   ::XDisplay* display = gfx::GetXDisplay();
@@ -56,8 +71,7 @@ std::vector<gfx::Display> GetFallbackDisplayList() {
   gfx::Display gfx_display(0, bounds_in_pixels);
   if (!gfx::Display::HasForceDeviceScaleFactor() &&
       !ui::IsDisplaySizeBlackListed(physical_size)) {
-    float device_scale_factor = GetDeviceScaleFactor(
-        width, physical_size.width());
+    float device_scale_factor = GetGlobalDeviceScaleFactor();
     DCHECK_LE(1.0f, device_scale_factor);
     gfx_display.SetScaleAndBounds(device_scale_factor, bounds_in_pixels);
   }
@@ -279,7 +293,9 @@ std::vector<gfx::Display> DesktopScreenX11::BuildDisplaysFromXRandRInfo() {
     has_work_area = true;
   }
 
+
   float device_scale_factor = 1.0f;
+  bool have_one = false;
   for (int i = 0; i < resources->noutput; ++i) {
     RROutput output_id = resources->outputs[i];
     XRROutputInfo* output_info =
@@ -307,13 +323,13 @@ std::vector<gfx::Display> DesktopScreenX11::BuildDisplaysFromXRandRInfo() {
       gfx::Display display(display_id, crtc_bounds);
 
       if (!gfx::Display::HasForceDeviceScaleFactor()) {
-        if (i == 0 && !ui::IsDisplaySizeBlackListed(
+        if (!have_one && !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);
+          device_scale_factor = GetGlobalDeviceScaleFactor();
           DCHECK_LE(1.0f, device_scale_factor);
+          have_one = true;
         }
         display.SetScaleAndBounds(device_scale_factor, crtc_bounds);
       }