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..6a575788d7316c23a3acf48036ca3841a58cafb6 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 kSyncMemoryPressureWarningCount[] = "sync.memory_warning_count";
+
+// Stores if sync shutdown cleanly.
+const char kSyncShutdownCleanly[] = "sync.shutdown_cleanly";
+
 }  // namespace prefs
 
 }  // namespace sync_driver