Create Field Trial param for whitelisting UKM Entries.

components/ukm/ukm_service.h

Index: components/ukm/ukm_service.h
diff --git a/components/ukm/ukm_service.h b/components/ukm/ukm_service.h
index 613658fbf5ac27e68888cb44304d00e99e523575..125d2ab801c23c80ff427f29d604e7941cd2ba07 100644
--- a/components/ukm/ukm_service.h
+++ b/components/ukm/ukm_service.h
@@ -117,6 +117,7 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
                            LogsUploadedOnlyWhenHavingSourcesOrEntries);
   FRIEND_TEST_ALL_PREFIXES(UkmServiceTest, MetricsProviderTest);
   FRIEND_TEST_ALL_PREFIXES(UkmServiceTest, PersistAndPurge);
+  FRIEND_TEST_ALL_PREFIXES(UkmServiceTest, WhitelistEntryTest);
 
   // Get a new UkmEntryBuilder object for the specified source ID and event,
   // which can get metrics added to.
@@ -150,6 +151,9 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
   // Add an entry to the UkmEntry list.
   void AddEntry(std::unique_ptr<UkmEntry> entry);
 
+  // Cache the list of whitelisted entries from the field trial parameter.
+  void StoreWhitelistedEntries();
+
   // A weak pointer to the PrefService used to read and write preferences.
   PrefService* pref_service_;
 
@@ -189,6 +193,9 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
   std::map<int32_t, std::unique_ptr<UkmSource>> sources_;
   std::vector<std::unique_ptr<UkmEntry>> entries_;
 
+  // Whitelisted Entry hashes, only the ones in this set will be recorded.
+  std::set<uint64_t> whitelisted_entry_hashes_;
+
   // Weak pointers factory used to post task on different threads. All weak
   // pointers managed by this factory have the same lifetime as UkmService.
   base::WeakPtrFactory<UkmService> self_ptr_factory_;