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 68d6ac67e1bdb8520b7736edaeef08a97a9fcf61..a8aa798fe68d90f141eddb96d70fddd403c5b291 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",
@@ -53,6 +55,7 @@ static_library("internal") {
     "//base",
     "//components/feature_engagement_tracker/public",
     "//components/keyed_service/core",
+    "//components/leveldb_proto",
   ]
 
   if (is_android) {
@@ -84,6 +87,7 @@ 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",
     "system_time_provider_unittest.cc",
   ]
@@ -91,6 +95,8 @@ source_set("unit_tests") {
   deps = [
     ":internal",
     "//base/test:test_support",
+    "//components/feature_engagement_tracker/internal/test:test_support",
+    "//components/leveldb_proto:test_support",
     "//testing/gtest",
   ]
 }