Move STATIC_HISTOGRAM_POINTER_GROUP into histogram_macros.h.

base/metrics/histogram_macros.h

Index: base/metrics/histogram_macros.h
diff --git a/base/metrics/histogram_macros.h b/base/metrics/histogram_macros.h
index 9e3caec3a349b2585eb1d82aea4f3b8e213f76da..ecfcc868d04c579b8b8eb3d367a88540be036f05 100644
--- a/base/metrics/histogram_macros.h
+++ b/base/metrics/histogram_macros.h
@@ -279,4 +279,34 @@
         base::CustomHistogram::FactoryGet(name, custom_ranges,                 \
             base::HistogramBase::kUmaTargetedHistogramFlag))
 
+// Support a collection of histograms, perhaps one for each entry in an
+// enumeration. This macro manages a block of pointers, adding to a specific
+// one by its index.

[Ilya Sherman, 2017/01/09 22:51:45]

nit: You've currently added this under the "Deprecated histogram macros"
section.  Please create a new section to contain this macro (unless there's
already an existing section that fits the bill; if so, then please use that.)

[tdresser, 2017/01/11 18:14:49]

Whoops. Fixed.

+//
+// A typical instantiation looks something like this:
+//  STATIC_HISTOGRAM_POINTER_GROUP(
+//      GetHistogramNameForIndex(histogram_index),
+//      histogram_index, MAXIMUM_HISTOGRAM_INDEX, Add(some_delta),
+//      base::Histogram::FactoryGet(
+//          GetHistogramNameForType(histogram_index),

[Ilya Sherman, 2017/01/09 22:51:45]

nit: s/Type/Index

[tdresser, 2017/01/11 18:14:49]

Done.

+//          MINIMUM_SAMPLE, MAXIMUM_SAMPLE, BUCKET_COUNT,
+//          base::HistogramBase::kUmaTargetedHistogramFlag));
+//
+// Though it seems inefficient to generate the name twice, the first
+// instance will be used only for DCHECK builds and the second will
+// execute only during the first access to the given index, after which
+// the pointer is cached and the name never needed again.
+#define STATIC_HISTOGRAM_POINTER_GROUP(constant_histogram_name, index,        \
+                                       constant_maximum,                      \
+                                       histogram_add_method_invocation,       \
+                                       histogram_factory_get_invocation)      \
+  do {                                                                        \
+    static base::subtle::AtomicWord atomic_histograms[constant_maximum];      \
+    DCHECK_LE(0, index);                                                      \
+    DCHECK_LT(index, constant_maximum);                                       \
+    HISTOGRAM_POINTER_USE(&atomic_histograms[index], constant_histogram_name, \
+                          histogram_add_method_invocation,                    \
+                          histogram_factory_get_invocation);                  \
+  } while (0)
+
 #endif  // BASE_METRICS_HISTOGRAM_MACROS_H_