Add support for persistent sparse histograms.

base/metrics/histogram.cc

Index: base/metrics/histogram.cc
diff --git a/base/metrics/histogram.cc b/base/metrics/histogram.cc
index ea4f8167fa28cc7098ce233cddbd7ed77fd23a77..79174b27151c354c976f4eef45ab8d36a1b27662 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -172,7 +172,7 @@ HistogramBase* Histogram::Factory::Build() {
     }
 
     // Try to create the histogram using a "persistent" allocator. As of
-    // 2015-01-14, the availability of such is controlled by a base::Feature
+    // 2016-02-25, the availability of such is controlled by a base::Feature

[grt (UTC plus 2), 2016/03/02 20:00:42]

updating this date repeatedly is cumbersome. wdyt about changing this to
"..allocator, the availability of which is..."

[bcwhite, 2016/03/02 22:32:43]

I don't plan to update it repeatedly.  I just updated it because I was putting
the same comment elsewhere and it seemed odd to have different dates in both.

     // that is off by default. If the allocator doesn't exist or if
     // allocating from it fails, code below will allocate the histogram from
     // the process heap.