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

ui/display/win/screen_win.cc

Index: ui/display/win/screen_win.cc
diff --git a/ui/display/win/screen_win.cc b/ui/display/win/screen_win.cc
index a4ef468fa5d8a3cf6fd7a0decd940dfb1704fac2..fa5fac0065c24bbe0931693be6a1c8d23f61daf7 100644
--- a/ui/display/win/screen_win.cc
+++ b/ui/display/win/screen_win.cc
@@ -8,9 +8,11 @@
 #include <shellscalingapi.h>
 
 #include <algorithm>
+#include <set>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/win/win_util.h"
 #include "ui/display/display.h"
 #include "ui/display/manager/display_layout.h"
@@ -411,6 +413,7 @@ void ScreenWin::Initialize() {
       new gfx::SingletonHwndObserver(
           base::Bind(&ScreenWin::OnWndProc, base::Unretained(this))));
   UpdateFromDisplayInfos(GetDisplayInfosFromSystem());
+  RecordDisplayScaleFactors();
 }
 
 MONITORINFOEX ScreenWin::MonitorInfoFromScreenPoint(
@@ -548,5 +551,21 @@ ScreenWinDisplay ScreenWin::GetScreenWinDisplayVia(Getter getter,
   return (g_screen_win_instance->*getter)(value);
 }
 
+void ScreenWin::RecordDisplayScaleFactors() const {
+  std::set<int> already_reported_scale_factors;

[robliao, 2016/07/20 00:58:10]

Go with an unordered_set or a vector (n is small enough so that vector will
outperform most hash based implementations on time and space).

n is also small enough where I would be okay with a vector of two passes: once
to gather the unique scale factors and the next to report them.

[Bret, 2016/07/20 18:59:42]

Done.

+  for (const auto& screen_win_display : screen_win_displays_) {
+    const float scale_factor =
+        screen_win_display.display().device_scale_factor();
+    // Clamp the reported value so that if it's wildly out-of-band we won't send
+    // it to the backend.
+    const int reported_scale = std::min(
+        std::max(base::checked_cast<int>(scale_factor * 100), 0), 1000);
+    if (!already_reported_scale_factors.count(reported_scale)) {
+      already_reported_scale_factors.insert(reported_scale);
+      UMA_HISTOGRAM_SPARSE_SLOWLY("UI.DeviceScale", reported_scale);
+    }
+  }
+}
+
 }  // namespace win
 }  // namespace display