[Cleanup] Clarify a histogram macro comment.

base/metrics/histogram_macros_internal.h

Index: base/metrics/histogram_macros_internal.h
diff --git a/base/metrics/histogram_macros_internal.h b/base/metrics/histogram_macros_internal.h
index 2deb9284a177fa26a4e3ecdd155addcb96422f6e..51195867123958ad971a2ba0d92c9b060c42ad56 100644
--- a/base/metrics/histogram_macros_internal.h
+++ b/base/metrics/histogram_macros_internal.h
@@ -96,9 +96,17 @@
         base::Histogram::FactoryGet(name, min, max, bucket_count, flag))
 
 // This is a helper macro used by other macros and shouldn't be used directly.
-// One additional bucket is created in the LinearHistogram for the illegal
-// values >= boundary_value so that mistakes in calling the UMA enumeration
-// macros can be detected.
+// For an enumeration with N items, recording values in the range [0, N + 1),

[rkaplow, 2017/02/22 20:24:20]

i think this should be [0, N], since I think you're not counting the 0th item?

ex N = 3, so N items

[0,1] [1,2] [2,3] [3,inf]
N+1 buckets

and then in the latter comment it should be
.., [N - 1, N)

and similar edits later, the maximal value is N-1 

[Ilya Sherman, 2017/02/22 23:31:07]

D'oh!  Off by one!  Good catch, Rob.  Done.

+// this macro creates a linear histogram with N + 1 buckets:
+//   [0, 1), [1, 2), ..., [N, N + 1), and an overflow bucket [N + 1, infinity).
+// Code should never emit to the overflow bucket; only to the other N buckets.
+// This allows future versions of Chrome to safely append new entries to the
+// enumeration. Otherwise, the histogram would have [N, infinity) as its
+// overflow bucket, and so the maximal value N would be emitted to this overflow
+// bucket. But, if an additional enumerated value were later added, the bucket
+// label for the value N would change to [N, N + 1), which would result in
+// different versions of Chrome using different bucket labels for identical
+// data.
 #define INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(name, sample, boundary, flag) \
   do {                                                                         \
     static_assert(                                                             \