Add a UMA histogram for a user's device scale at startup.

ui/display/win/dpi.cc

Index: ui/display/win/dpi.cc
diff --git a/ui/display/win/dpi.cc b/ui/display/win/dpi.cc
index 778d83b0d468dc2cd082e98831514f6fe6b3ddb1..60805d066825beda5d8186737556dd5f3cb9aca4 100644
--- a/ui/display/win/dpi.cc
+++ b/ui/display/win/dpi.cc
@@ -4,8 +4,12 @@
 
 #include "ui/display/win/dpi.h"
 
+#include <algorithm>
+
 #include <windows.h>
 
+#include "base/metrics/histogram.h"

[Ilya Sherman, 2016/07/16 01:31:24]

nit: You shouldn't need this include -- just the sparse_histogram include.

[Bret, 2016/07/18 17:41:48]

Done.

+#include "base/metrics/sparse_histogram.h"
 #include "base/win/scoped_hdc.h"
 #include "ui/display/display.h"
 
@@ -18,6 +22,8 @@ const float kDefaultDPI = 96.f;
 
 float g_device_scale_factor = 0.f;
 
+void RecordDeviceScale();
+
 gfx::Size GetDPI() {
   static int dpi_x = 0;
   static int dpi_y = 0;
@@ -31,14 +37,22 @@ gfx::Size GetDPI() {
     // to all screens.
     dpi_x = GetDeviceCaps(screen_dc, LOGPIXELSX);
     dpi_y = GetDeviceCaps(screen_dc, LOGPIXELSY);
+    RecordDeviceScale();
   }
   return gfx::Size(dpi_x, dpi_y);
 }
 
 float GetUnforcedDeviceScaleFactor() {
-  return g_device_scale_factor
-      ? g_device_scale_factor
-      : GetScalingFactorFromDPI(GetDPI().width());
+  return g_device_scale_factor ? g_device_scale_factor
+                               : GetScalingFactorFromDPI(GetDPI().width());
+}
+
+void RecordDeviceScale() {
+  // Clamp the reported value so that if it's wildly out-of-band we won't send
+  // it to the backend.
+  const int scale = std::min(
+      std::max(std::floor(GetUnforcedDeviceScaleFactor() * 100), 0.f), 1000.f);

[Ilya Sherman, 2016/07/16 01:31:24]

nit: Somewhere, please add some explicit code to convert from float to int,
probably using base::checked_cast<>.

[Bret, 2016/07/18 17:41:48]

Done.

+  UMA_HISTOGRAM_SPARSE_SLOWLY("UI.DeviceScale", scale);
 }
 
 }  // namespace