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

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

 // Copyright (c) 2013 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 "chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h"
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 36 matching lines @@
 
 #if defined(USE_OZONE) || defined(USE_X11)
 #include "ui/events/devices/input_device_event_observer.h"
 #include "ui/events/devices/input_device_manager.h"
 #endif  // defined(USE_OZONE) || defined(USE_X11)
 
 #if defined(OS_WIN)
 #include "base/win/windows_version.h"
 #include "chrome/browser/shell_integration_win.h"
 #include "chrome/installer/util/google_update_settings.h"
+#include "ui/display/win/dpi.h"
 #endif  // defined(OS_WIN)
 
 namespace {
 
 enum UMALinuxGlibcVersion {
   UMA_LINUX_GLIBC_NOT_PARSEABLE,
   UMA_LINUX_GLIBC_UNKNOWN,
   UMA_LINUX_GLIBC_2_11,
   // To log newer versions, just update tools/metrics/histograms/histograms.xml.
 };
@@ skipping 197 matching lines @@
     state = UMA_TOUCH_EVENTS_DISABLED;
   } else {
     NOTREACHED();
     return;
   }
 
   UMA_HISTOGRAM_ENUMERATION("Touchscreen.TouchEventsEnabled", state,
                             UMA_TOUCH_EVENTS_STATE_COUNT);
 }
 
+#if defined(OS_WIN)
+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(display::win::GetUnforcedDeviceScaleFactor() * 100),
+               0.f),
+      1000.f);

[Ilya Sherman, 2016/07/16 00:43:10]

nit: I think you want 0 and 1000 instead of 0.f and 1000.f, since the final
result is an int.

[Bret, 2016/07/16 01:22:35]

std::floor returns a float and std::min/max require their arguments to be the
same type. Doing this seemed slightly less silly than doing
static_cast<int>(std::floor(etc)) because the value is still unclamped.

+  UMA_HISTOGRAM_SPARSE_SLOWLY("UI.DeviceScale", scale);
+}
+#endif

[Ilya Sherman, 2016/07/16 00:43:10]

Could you please find a different location for recording this metric? 
chrome_browser_main_extra_parts_metrics isn't intended for reporting all kinds
of unrelated metrics; otherwise, it'd end up an overwhelmingly large, hairy
file.

[Bret, 2016/07/16 01:22:35]

Done. Moved into dpi.cc initialization.

+
 #if defined(USE_OZONE) || defined(USE_X11)
 
 // Asynchronously records the touch event state when the ui::DeviceDataManager
 // completes a device scan.
 class AsynchronousTouchEventStateRecorder
     : public ui::InputDeviceEventObserver {
  public:
   AsynchronousTouchEventStateRecorder();
   ~AsynchronousTouchEventStateRecorder() override;
 
@@ skipping 65 matching lines @@
 #if defined(OS_MACOSX)
   RecordMacMetrics();
 #endif  // defined(OS_MACOSX)
 
   constexpr base::TimeDelta kStartupMetricsGatheringDelay =
       base::TimeDelta::FromSeconds(45);
   content::BrowserThread::GetBlockingPool()->PostDelayedTask(
       FROM_HERE, base::Bind(&RecordStartupMetricsOnBlockingPool),
       kStartupMetricsGatheringDelay);
 #if defined(OS_WIN)
+  RecordDeviceScale();
   content::BrowserThread::PostDelayedTask(
       content::BrowserThread::IO, FROM_HERE,
       base::Bind(&shell_integration::win::RecordIsPinnedToTaskbarHistogram),
       kStartupMetricsGatheringDelay);
 #endif  // defined(OS_WIN)
 
   display_count_ = display::Screen::GetScreen()->GetNumDisplays();
   UMA_HISTOGRAM_COUNTS_100("Hardware.Display.Count.OnStartup", display_count_);
   display::Screen::GetScreen()->AddObserver(this);
   is_screen_observer_ = true;
@@ skipping 25 matching lines @@
   }
 }
 
 namespace chrome {
 
 void AddMetricsExtraParts(ChromeBrowserMainParts* main_parts) {
   main_parts->AddParts(new ChromeBrowserMainExtraPartsMetrics());
 }
 
 }  // namespace chrome