Support saving browser metrics to disk and reading them during next run.

base/metrics/persistent_histogram_allocator.h

Index: base/metrics/persistent_histogram_allocator.h
diff --git a/base/metrics/persistent_histogram_allocator.h b/base/metrics/persistent_histogram_allocator.h
index 7f46dc9664bd6a45d3c010f8cebd100e9ddce25a..d29d92d18a915ca0f53399ced390ca931be3ba5b 100644
--- a/base/metrics/persistent_histogram_allocator.h
+++ b/base/metrics/persistent_histogram_allocator.h
@@ -398,6 +398,16 @@ class BASE_EXPORT GlobalHistogramAllocator
   // in other ways.
   static std::unique_ptr<GlobalHistogramAllocator> ReleaseForTesting();
 
+  // Stores a pathname to which the contents of this allocator should be saved
+  // in order to persist the data for a later use.
+  void SetPersistentLocation(const FilePath& location);
+
+  // Writes the internal data to a previously set location. This is generally
+  // called when a process is exiting from a section of code that may not know
+  // the filesystem. The data is written in an atomic manner. The return value
+  // indicates success.
+  bool WriteToPersistentLocation();
+
  private:
   friend class StatisticsRecorder;
 
@@ -416,6 +426,9 @@ class BASE_EXPORT GlobalHistogramAllocator
   // iterator to continue the work.
   Iterator import_iterator_;
 
+  // The location to which the data should be persisted.
+  FilePath persistent_location_;
+
   DISALLOW_COPY_AND_ASSIGN(GlobalHistogramAllocator);
 };