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

base/metrics/persistent_histogram_allocator.cc

Index: base/metrics/persistent_histogram_allocator.cc
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 20f64336016ae7acf269c3faa25ba3e8fc585425..9c58fa811da8c25755c3a85f320e0ac64f30d8db 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -713,6 +713,10 @@ void GlobalHistogramAllocator::SetPersistentLocation(const FilePath& location) {
   persistent_location_ = location;
 }
 
+const FilePath& GlobalHistogramAllocator::GetPersistentLocation() const {
+  return persistent_location_;
+}
+
 bool GlobalHistogramAllocator::WriteToPersistentLocation() {
 #if defined(OS_NACL)
   // NACL doesn't support file operations, including ImportantFileWriter.