Add metrics for the number of attached displays.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h

Index: chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
diff --git a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
index 0e4fdbb9efc7aaad7612cd469ab0ee18e957b360..b1b72f078a48d224ee38babe4898888cf1990a17 100644
--- a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
+++ b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
@@ -10,6 +10,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chrome_browser_main_extra_parts.h"
 
+class ChromeBrowserMainExtraPartsMetricsMac;
 class ChromeBrowserMainParts;
 
 namespace chrome {
@@ -27,6 +28,12 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
   virtual void PostBrowserStart() OVERRIDE;
 
  private:
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+  // This member records OSX specific metrics.
+  base::scoped_ptr<ChromeBrowserMainExtraPartsMetricsMac>

[Robert Sesek, 2014/10/08 19:25:20]

I think this could be an OS_MACOSX-specific method instead,
AddMacMonitorMetrics() that is called in the right override instead. The
implementation can then live in the .mm file. The extra C++ class seems
heavyweight.

[erikchen, 2014/10/08 22:49:19]

I removed all of the ObjC and OSX specific code from the CL. Theoretically, this
CL could be refactored so that no new files are required. If we wanted to
collect these metrics on all platforms, moving all the code into the existing
files seems like the best way forward.

I am operating under the assumption that we should only add metrics that we
directly intend on using, and that we shouldn't add these metrics for the other
platforms. In that case, separating the code (~80 lines total) into OS specific
files seems like the right way forward. 

Your suggestion works well for the metric OSX.DisplayCount.StartUp, but doesn't
work well for the metric OSX.DisplayCount.Changed, which requires
adding/removing observers, changing the inheritance hierarchy, etc.

I don't care too strongly about this, so if you still prefer a reduction in the
number of new files, just let me know.

+      extra_parts_metrics_mac_;
+#endif  // defined(OS_MACOSX) && !defined(OS_IOS)
+
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetrics);
 };