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

ui/display/win/dpi.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 
 namespace display {
 namespace win {
 
 namespace {
 
 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;
   static bool should_initialize = true;
 
   if (should_initialize) {
     should_initialize = false;
     base::win::ScopedGetDC screen_dc(NULL);
     // This value is safe to cache for the life time of the app since the
     // user must logout to change the DPI setting. This value also applies
     // 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
 
 void SetDefaultDeviceScaleFactor(float scale) {
   DCHECK_NE(0.f, scale);
   g_device_scale_factor = scale;
 }
 
 float GetDPIScale() {
@@ skipping 13 matching lines @@
 
 int GetSystemMetricsInDIP(int metric) {
   // The system metrics always reflect the system DPI, not whatever scale we've
   // forced or decided to use.
   return static_cast<int>(
       std::round(GetSystemMetrics(metric) / GetUnforcedDeviceScaleFactor()));
 }
 
 }  // namespace win
 }  // namespace display