Add support for persistent sparse histograms.

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 7f363133802a0219f6ca3f029d53e4ae8101b881..b8767377ab167187cbfca354030afaa66153e0b2 100644
--- a/base/metrics/sparse_histogram.h
+++ b/base/metrics/sparse_histogram.h
@@ -51,6 +51,7 @@ namespace base {
     } while (0)
 
 class HistogramSamples;
+class PersistentMemoryAllocator;
 
 class BASE_EXPORT SparseHistogram : public HistogramBase {
  public:
@@ -58,6 +59,13 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // new one.
   static HistogramBase* FactoryGet(const std::string& name, int32_t flags);
 
+  // Create a histogram using data in persistent storage.
+  static scoped_ptr<HistogramBase> PersistentCreate(
+      PersistentMemoryAllocator* allocator,
+      const std::string& name,
+      HistogramSamples::Metadata* meta,
+      HistogramSamples::Metadata* logged_meta);
+
   ~SparseHistogram() override;
 
   // HistogramBase implementation:
@@ -83,6 +91,11 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // Clients should always use FactoryGet to create SparseHistogram.
   explicit SparseHistogram(const std::string& name);
 
+  SparseHistogram(PersistentMemoryAllocator* allocator,
+                  const std::string& name,
+                  HistogramSamples::Metadata* meta,
+                  HistogramSamples::Metadata* logged_meta);
+
   friend BASE_EXPORT HistogramBase* DeserializeHistogramInfo(
       base::PickleIterator* iter);
   static HistogramBase* DeserializeInfoImpl(base::PickleIterator* iter);
@@ -107,8 +120,8 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // Protects access to |samples_|.
   mutable base::Lock lock_;
 
-  SampleMap samples_;
-  SampleMap logged_samples_;
+  scoped_ptr<HistogramSamples> samples_;
+  scoped_ptr<HistogramSamples> logged_samples_;
 
   DISALLOW_COPY_AND_ASSIGN(SparseHistogram);
 };