[Sync] Refactor passphrase state handling

chrome/browser/sync/glue/sync_backend_host.h

Index: chrome/browser/sync/glue/sync_backend_host.h
diff --git a/chrome/browser/sync/glue/sync_backend_host.h b/chrome/browser/sync/glue/sync_backend_host.h
index 65aaa0454066ab273cf494b3592433e0da40f3c4..0786a6c2aee664c9930135e1490319c512e0a2d4 100644
--- a/chrome/browser/sync/glue/sync_backend_host.h
+++ b/chrome/browser/sync/glue/sync_backend_host.h
@@ -454,6 +454,11 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // Invoked when sync finishes encrypting new datatypes.
   void NotifyEncryptionComplete();
 
+  // Invoked when the passphrase state has changed. Caches the passphrase state
+  // for later use on the UI thread.
+  void HandlePassphraseStateChangedOnFrontendLoop(
+      syncer::PassphraseState state);
+
   void HandleStopSyncingPermanentlyOnFrontendLoop();
 
   // Dispatched to from OnConnectionStatusChange to handle updating
@@ -522,6 +527,12 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // were cached.
   sync_pb::EncryptedData cached_pending_keys_;
 
+  // The state of the passphrase required to decrypt the bag of encryption keys
+  // in the nigori node. Updated whenever a new nigori node arrives or the user
+  // manually changes their passphrase state. Cached so we can synchronously
+  // check it from the UI thread.
+  syncer::PassphraseState cached_passphrase_state_;
+
   // UI-thread cache of the last SyncSessionSnapshot received from syncapi.
   syncer::sessions::SyncSessionSnapshot last_snapshot_;