Add UMA data for memory pressure warning received for sync

components/sync_driver/pref_names.cc

Index: components/sync_driver/pref_names.cc
diff --git a/components/sync_driver/pref_names.cc b/components/sync_driver/pref_names.cc
index b167a0b4165b3164a41e55edbde2a80cff0473d3..f3f75a91691afa3aa471dd29bb8a4ff889c688f4 100644
--- a/components/sync_driver/pref_names.cc
+++ b/components/sync_driver/pref_names.cc
@@ -101,6 +101,12 @@ const char kSyncFirstSyncTime[] = "sync.first_sync_time";
 // that we only want to use once.
 const char kSyncPassphrasePrompted[] = "sync.passphrase_prompted";
 
+// Stores how many times received MEMORY_PRESSURE_LEVEL_CRITICAL
+const char kSyncMemoryPressureReceivings[] = "sync.memory_warning_receivings";
+
+// Stores if sync shutdown correctly (not crash)
+const char kSyncShutdownUnnormal[] = "sync.shutdown_forced";
+
 }  // namespace prefs
 
 }  // namespace sync_driver