Refactor histogram_persistence to be a class.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 61c1bd7e2ceff3fdc876018d1357b6798ae943c1..9c59ee39f29ce62658efd795f6154fc345cac7e9 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -517,14 +517,14 @@ component("base") {
     "metrics/histogram_delta_serialization.h",
     "metrics/histogram_flattener.h",
     "metrics/histogram_macros.h",
-    "metrics/histogram_persistence.cc",
-    "metrics/histogram_persistence.h",
     "metrics/histogram_samples.cc",
     "metrics/histogram_samples.h",
     "metrics/histogram_snapshot_manager.cc",
     "metrics/histogram_snapshot_manager.h",
     "metrics/metrics_hashes.cc",
     "metrics/metrics_hashes.h",
+    "metrics/persistent_histogram_allocator.cc",
+    "metrics/persistent_histogram_allocator.h",
     "metrics/persistent_memory_allocator.cc",
     "metrics/persistent_memory_allocator.h",
     "metrics/sample_map.cc",
@@ -1655,6 +1655,7 @@ test("base_unittests") {
     "metrics/histogram_snapshot_manager_unittest.cc",
     "metrics/histogram_unittest.cc",
     "metrics/metrics_hashes_unittest.cc",
+    "metrics/persistent_histogram_allocator_unittest.cc",
     "metrics/persistent_memory_allocator_unittest.cc",
     "metrics/sample_map_unittest.cc",
     "metrics/sample_vector_unittest.cc",