Modifies the threshold for hidpi displays.

ui/display/display_util.cc

Index: ui/display/display_util.cc
diff --git a/ui/display/display_util.cc b/ui/display/display_util.cc
index a51b19af886bb4f3f1d43640d1fcac999a67a2dc..aeaeed939a012be95236d0301219dde5d880529f 100644
--- a/ui/display/display_util.cc
+++ b/ui/display/display_util.cc
@@ -20,6 +20,21 @@ const int kInvalidDisplaySizeList[][2] = {
   {160, 100},
 };
 
+// The DPI threshold to detect high density screen.
+// Higher DPI than this will use device_scale_factor=2.
+const unsigned int kHighDensityDPIThresholdSmall = 170;
+
+// The HiDPI threshold for large (usually external) monitors. Lower threshold
+// makes sense for large monitors, because such monitors should be located
+// farther from the user's face usually. See http://crbug.com/348279
+const unsigned int kHighDensityDPIThresholdLarge = 150;
+
+// The width threshold in mm for "large" monitors.
+const int kLargeDisplayWidthThresholdMM = 500;
+
+// 1 inch in mm.
+const float kInchInMm = 25.4f;
+
 }  // namespace
 
 bool IsDisplaySizeBlackListed(const gfx::Size& physical_size) {
@@ -40,4 +55,17 @@ bool IsDisplaySizeBlackListed(const gfx::Size& physical_size) {
   return false;
 }
 
+float GetScaleFactor(const gfx::Size& physical_size_in_mm,
+                     const gfx::Size& screen_size_in_pixels) {
+  if (IsDisplaySizeBlackListed(physical_size_in_mm))
+    return 1.0f;
+
+  const unsigned int dpi = (kInchInMm * screen_size_in_pixels.width() /
+                            physical_size_in_mm.width());
+  const unsigned int threshold =
+      (physical_size_in_mm.width() >= kLargeDisplayWidthThresholdMM) ?
+      kHighDensityDPIThresholdLarge : kHighDensityDPIThresholdSmall;
+  return (dpi > threshold) ? 2.0f : 1.0f;
+}
+
 }  // namespace ui