Add SampleMap and use it in SparseHistogram

base/metrics/sparse_histogram.cc

Index: base/metrics/sparse_histogram.cc
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index 05a68b53540a1f5daf23c62c8ac6b6faa94e4006..b8f097fd14a1e49dcd0d359e04542ea31ebcbfb9 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -22,13 +22,13 @@ HistogramBase* SparseHistogram::FactoryGet(const string& name,
 SparseHistogram::~SparseHistogram() {}
 
 void SparseHistogram::Add(Sample value) {
-  base::AutoLock auto_lock(lock_);
-  samples_[value]++;
+  samples_.Accumulate(value, 1);
 }
 
-void SparseHistogram::SnapshotSample(std::map<Sample, Count>* samples) const {
-  base::AutoLock auto_lock(lock_);
-  *samples = samples_;
+scoped_ptr<SampleMap> SparseHistogram::SnapshotSamples() const {
+  scoped_ptr<SampleMap> snapshot(new SampleMap());
+  snapshot->Add(samples_);
+  return snapshot.Pass();
 }
 
 void SparseHistogram::WriteHTMLGraph(string* output) const {