Update from https://crrev.com/333737

base/metrics/sample_map.cc

Index: base/metrics/sample_map.cc
diff --git a/base/metrics/sample_map.cc b/base/metrics/sample_map.cc
index 42468cb57f31816fa6a5fbd0c2c5e734a86c6773..f2540a4d00bf08ee5b6548692e80ce9bbb155462 100644
--- a/base/metrics/sample_map.cc
+++ b/base/metrics/sample_map.cc
@@ -6,8 +6,6 @@
 
 #include "base/logging.h"
 
-using std::map;
-
 namespace base {
 
 typedef HistogramBase::Count Count;
@@ -24,7 +22,7 @@ void SampleMap::Accumulate(Sample value, Count 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;
@@ -32,10 +30,8 @@ Count SampleMap::GetCount(Sample value) const {
 
 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;
 }
@@ -53,14 +49,17 @@ bool SampleMap::AddSubtractImpl(SampleCountIterator* iter,
     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() {}
 
@@ -70,7 +69,8 @@ bool SampleMapIterator::Done() const {
 
 void SampleMapIterator::Next() {
   DCHECK(!Done());
-  iter_++;
+  ++iter_;
+  SkipEmptyBuckets();
 }
 
 void SampleMapIterator::Get(Sample* min, Sample* max, Count* count) const {
@@ -83,4 +83,10 @@ void SampleMapIterator::Get(Sample* min, Sample* max, Count* count) const {
     *count = iter_->second;
 }
 
+void SampleMapIterator::SkipEmptyBuckets() {
+  while (!Done() && iter_->second == 0) {
+    ++iter_;
+  }
+}
+
 }  // namespace base