Add metrics for the number of attached displays.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h

 // 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.
 
 #ifndef CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_
 #define CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chrome_browser_main_extra_parts.h"
 
+class ChromeBrowserMainExtraPartsMetricsMac;
 class ChromeBrowserMainParts;
 
 namespace chrome {
 void AddMetricsExtraParts(ChromeBrowserMainParts* main_parts);
 }
 
 class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
  public:
   ChromeBrowserMainExtraPartsMetrics();
   virtual ~ChromeBrowserMainExtraPartsMetrics();
 
   // Overridden from ChromeBrowserMainExtraParts:
   virtual void PreProfileInit() OVERRIDE;
   virtual void PreBrowserStart() OVERRIDE;
   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);
 };
 
 #endif  // CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_