Add a PersistentStore to FeatureEngagementTracker

components/feature_engagement_tracker/internal/BUILD.gn

Index: components/feature_engagement_tracker/internal/BUILD.gn
diff --git a/components/feature_engagement_tracker/internal/BUILD.gn b/components/feature_engagement_tracker/internal/BUILD.gn
index 21a6e5ac5f260afc169fb93314e7a3061bd89505..4dee1a03b082d76f11af2d0ec028342c57a4a50b 100644
--- a/components/feature_engagement_tracker/internal/BUILD.gn
+++ b/components/feature_engagement_tracker/internal/BUILD.gn
@@ -36,6 +36,8 @@ static_library("internal") {
     "never_storage_validator.h",
     "once_condition_validator.cc",
     "once_condition_validator.h",
+    "persistent_store.cc",
+    "persistent_store.h",
     "single_invalid_configuration.cc",
     "single_invalid_configuration.h",
     "storage_validator.h",
@@ -80,12 +82,15 @@ source_set("unit_tests") {
     "never_condition_validator_unittest.cc",
     "never_storage_validator_unittest.cc",
     "once_condition_validator_unittest.cc",
+    "persistent_store_unittest.cc",
     "single_invalid_configuration_unittest.cc",
   ]
 
   deps = [
     ":internal",
     "//base/test:test_support",
+    "//components/feature_engagement_tracker/internal/test:test_support",
+    "//components/leveldb_proto:test_support",
     "//testing/gtest",
   ]
 }