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 6d790e1a09ab4f9d689e4c47d0fe1d614def0c3e..f2da4c646463aa45f1328472ccd88734237493e4 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -812,6 +812,10 @@ void GlobalHistogramAllocator::SetPersistentLocation(const FilePath& location) {
   persistent_location_ = location;
 }
 
+const FilePath& GlobalHistogramAllocator::GetPersistentLocation() {
+  return persistent_location_;
+}
+
 bool GlobalHistogramAllocator::WriteToPersistentLocation() {
 #if defined(OS_NACL)
   // NACL doesn't support file operations, including ImportantFileWriter.