Add support for persistent sparse histograms.

base/metrics/histogram.cc

 // Copyright (c) 2012 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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 // See header file for details and examples.
 
 #include "base/metrics/histogram.h"
@@ skipping 154 matching lines @@
     // cases (such as with a CustomHistogram), they are calculated dynamically
     // at run-time. In the latter case, those ctor parameters are zero and
     // the results extracted from the result of CreateRanges().
     if (bucket_count_ == 0) {
       bucket_count_ = static_cast<uint32_t>(registered_ranges->bucket_count());
       minimum_ = registered_ranges->range(1);
       maximum_ = registered_ranges->range(bucket_count_ - 1);
     }
 
     // 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.
     PersistentMemoryAllocator::Reference histogram_ref = 0;
     HistogramBase* tentative_histogram = nullptr;
     PersistentMemoryAllocator* allocator =
         GetPersistentHistogramMemoryAllocator();
     if (allocator) {
       flags_ |= HistogramBase::kIsPersistent;
       tentative_histogram = AllocatePersistentHistogram(
@@ skipping 969 matching lines @@
     Sample sample = custom_ranges[i];
     if (sample < 0 || sample > HistogramBase::kSampleType_MAX - 1)
       return false;
     if (sample != 0)
       has_valid_range = true;
   }
   return has_valid_range;
 }
 
 }  // namespace base