Improve efficiency of 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 f0296cbb1e1b1a2988020339df236943dc5b6986..da373a287a5c9e9c2cffabc239714093aed46477 100644
--- a/base/metrics/sparse_histogram.h
+++ b/base/metrics/sparse_histogram.h
@@ -51,7 +51,7 @@ namespace base {
     } while (0)
 
 class HistogramSamples;
-class PersistentMemoryAllocator;
+class PersistentHistogramAllocator;
 
 class BASE_EXPORT SparseHistogram : public HistogramBase {
  public:
@@ -61,7 +61,7 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
 
   // Create a histogram using data in persistent storage.
   static std::unique_ptr<HistogramBase> PersistentCreate(
-      PersistentMemoryAllocator* allocator,
+      PersistentHistogramAllocator* allocator,
       const std::string& name,
       HistogramSamples::Metadata* meta,
       HistogramSamples::Metadata* logged_meta);
@@ -91,7 +91,7 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // Clients should always use FactoryGet to create SparseHistogram.
   explicit SparseHistogram(const std::string& name);
 
-  SparseHistogram(PersistentMemoryAllocator* allocator,
+  SparseHistogram(PersistentHistogramAllocator* allocator,
                   const std::string& name,
                   HistogramSamples::Metadata* meta,
                   HistogramSamples::Metadata* logged_meta);