Support experiment to store persistent metrics in memory-mapped file.

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 5464da6a573148a7096870f1835af1d7f349c0e5..a89197031fb78cf073561abc9aedf06945469592 100644
--- a/base/metrics/persistent_histogram_allocator.h
+++ b/base/metrics/persistent_histogram_allocator.h
@@ -439,6 +439,10 @@ class BASE_EXPORT GlobalHistogramAllocator
   // in order to persist the data for a later use.
   void SetPersistentLocation(const FilePath& location);
 
+  // Retrieves a previously set pathname to which the contents of this allocator
+  // are to be saved.
+  const FilePath& GetPersistentLocation();
+
   // 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