Report timing statistics for audio controller methods via UMA.

media/base/scoped_histogram_timer.h

Index: media/base/scoped_histogram_timer.h
diff --git a/media/base/scoped_histogram_timer.h b/media/base/scoped_histogram_timer.h
new file mode 100644
index 0000000000000000000000000000000000000000..728e49f815359eece7ec66af77aaf311f766eabf
--- /dev/null
+++ b/media/base/scoped_histogram_timer.h
@@ -0,0 +1,31 @@
+// Copyright 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 MEDIA_BASE_SCOPED_HISTOGRAM_TIMER_H_
+#define MEDIA_BASE_SCOPED_HISTOGRAM_TIMER_H_
+
+#include "base/metrics/histogram.h"
+#include "base/time.h"
+
+// Scoped class which logs its time on this earth as a UMA statistic.  Must be
+// a #define macro since UMA macros prevent variables as names.  The nested
+// macro is necessary to expand __COUNTER__ to an actual value.
+#define SCOPED_UMA_HISTOGRAM_TIMER(name) \
+  SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, __COUNTER__)
+
+#define SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, key) \
+  SCOPED_UMA_HISTOGRAM_TIMER_UNIQUE(name, key)
+
+#define SCOPED_UMA_HISTOGRAM_TIMER_UNIQUE(name, key) \
+  class ScopedHistogramTimer##key { \
+   public: \
+    ScopedHistogramTimer##key() : constructed_(base::TimeTicks::Now()) {} \
+    ~ScopedHistogramTimer##key() { \
+      UMA_HISTOGRAM_TIMES(name, base::TimeTicks::Now() - constructed_); \

[Alexei Svitkine (slow), 2013/05/14 15:55:43]

Please extract base::TimeTicks::Now() - contructed_ into a local variable,
before invoking a macro on it.

Otherwise, depending on the implementation of the macro, the expression may be
evaluated multiple times, which is bad when it's calling a function. (I'm not
sure if it's actually a problem with this macro, but it's better be safe even if
it isn't, in case the implementation changes).

[jar (doing other things), 2013/05/14 16:19:22]

Good suggestion.  In this case the given arg is the sample, and is evaluated
exactly once (before being passed to a method).

It is still probably a good enough habit that it should done (so no one else
worries).

[DaleCurtis, 2013/05/15 01:30:31]

Done.

+    } \
+   private: \
+    base::TimeTicks constructed_; \
+  } scoped_histogram_timer_##key
+
+#endif  // MEDIA_BASE_SCOPED_HISTOGRAM_TIMER_H_