Clean up sparse_histogram.h header

base/metrics/histogram_macros_internal.h

 // Copyright 2016 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 BASE_METRICS_HISTOGRAM_MACROS_INTERNAL_H_
 #define BASE_METRICS_HISTOGRAM_MACROS_INTERNAL_H_
 
 #include "base/atomicops.h"
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/time/time.h"
 
 // This is for macros internal to base/metrics. They should not be used outside
 // of this directory. For writing to UMA histograms, see histogram_macros.h.
 
 // TODO(rkaplow): Improve commenting of these methods.
 
 //------------------------------------------------------------------------------
 // Histograms are often put in areas where they are called many many times, and
 // performance is critical.  As a result, they are designed to have a very low
@@ skipping 99 matching lines @@
         UMA_HISTOGRAM_LONG_TIMES_100(name, elapsed);                           \
       } else {                                                                 \
         UMA_HISTOGRAM_TIMES(name, elapsed);                                    \
       }                                                                        \
     }                                                                          \
    private:                                                                    \
     base::TimeTicks constructed_;                                              \
   } scoped_histogram_timer_##key
 
 #endif  // BASE_METRICS_HISTOGRAM_MACROS_INTERNAL_H_
+
+// Macro for sparse histogram.
+// The implementation is more costly to add values to, and each value
+// stored has more overhead, compared to the other histogram types. However it
+// may be more efficient in memory if the total number of sample values is small
+// compared to the range of their values.
+#define INTERNAL_HISTOGRAM_SPARSE_SLOWLY(name, sample)                         \
+    do {                                                                       \
+      base::HistogramBase* histogram = base::SparseHistogram::FactoryGet(      \
+          name, base::HistogramBase::kUmaTargetedHistogramFlag);               \
+      histogram->Add(sample);                                                  \
+    } while (0)