Merge multiple histogram snapshots into single one for reporting.

base/metrics/sparse_histogram.h

Index: base/metrics/sparse_histogram.h
diff --git a/base/metrics/sparse_histogram.h b/base/metrics/sparse_histogram.h
index 04909c0859a5e948366fde87fd88faca142397e2..7f363133802a0219f6ca3f029d53e4ae8101b881 100644
--- a/base/metrics/sparse_histogram.h
+++ b/base/metrics/sparse_histogram.h
@@ -71,6 +71,7 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   void AddSamples(const HistogramSamples& samples) override;
   bool AddSamplesFromPickle(base::PickleIterator* iter) override;
   scoped_ptr<HistogramSamples> SnapshotSamples() const override;
+  scoped_ptr<HistogramSamples> SnapshotDelta() override;
   void WriteHTMLGraph(std::string* output) const override;
   void WriteAscii(std::string* output) const override;
 
@@ -107,6 +108,7 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   mutable base::Lock lock_;
 
   SampleMap samples_;
+  SampleMap logged_samples_;
 
   DISALLOW_COPY_AND_ASSIGN(SparseHistogram);
 };