Update from https://crrev.com/333737

base/metrics/sample_map.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.
 
 #include "base/metrics/sample_map.h"
 
 #include "base/logging.h"
 
-using std::map;
-
 namespace base {
 
 typedef HistogramBase::Count Count;
 typedef HistogramBase::Sample Sample;
 
 SampleMap::SampleMap() {}
 
 SampleMap::~SampleMap() {}
 
 void SampleMap::Accumulate(Sample value, Count count) {
   sample_counts_[value] += count;
   IncreaseSum(count * value);
   IncreaseRedundantCount(count);
 }
 
 Count SampleMap::GetCount(Sample value) const {
-  map<Sample, Count>::const_iterator it = sample_counts_.find(value);
+  std::map<Sample, Count>::const_iterator it = sample_counts_.find(value);
   if (it == sample_counts_.end())
     return 0;
   return it->second;
 }
 
 Count SampleMap::TotalCount() const {
   Count count = 0;
-  for (map<Sample, Count>::const_iterator it = sample_counts_.begin();
-       it != sample_counts_.end();
-       ++it) {
-    count += it->second;
+  for (const auto& entry : sample_counts_) {
+    count += entry.second;
   }
   return count;
 }
 
 scoped_ptr<SampleCountIterator> SampleMap::Iterator() const {
   return scoped_ptr<SampleCountIterator>(new SampleMapIterator(sample_counts_));
 }
 
 bool SampleMap::AddSubtractImpl(SampleCountIterator* iter,
                                 HistogramSamples::Operator op) {
   Sample min;
   Sample max;
   Count count;
   for (; !iter->Done(); iter->Next()) {
     iter->Get(&min, &max, &count);
     if (min + 1 != max)
       return false;  // SparseHistogram only supports bucket with size 1.
-    sample_counts_[min] += (op ==  HistogramSamples::ADD) ? count : -count;
+
+    sample_counts_[min] += (op == HistogramSamples::ADD) ? count : -count;
   }
   return true;
 }
 
 SampleMapIterator::SampleMapIterator(const SampleToCountMap& sample_counts)
     : iter_(sample_counts.begin()),
-      end_(sample_counts.end()) {}
+      end_(sample_counts.end()) {
+  SkipEmptyBuckets();
+}
 
 SampleMapIterator::~SampleMapIterator() {}
 
 bool SampleMapIterator::Done() const {
   return iter_ == end_;
 }
 
 void SampleMapIterator::Next() {
   DCHECK(!Done());
-  iter_++;
+  ++iter_;
+  SkipEmptyBuckets();
 }
 
 void SampleMapIterator::Get(Sample* min, Sample* max, Count* count) const {
   DCHECK(!Done());
   if (min != NULL)
     *min = iter_->first;
   if (max != NULL)
     *max = iter_->first + 1;
   if (count != NULL)
     *count = iter_->second;
 }
 
+void SampleMapIterator::SkipEmptyBuckets() {
+  while (!Done() && iter_->second == 0) {
+    ++iter_;
+  }
+}
+
 }  // namespace base