sync: Gracefully handle very early shutdown

sync/internal_api/public/sync_manager.h

Index: sync/internal_api/public/sync_manager.h
diff --git a/sync/internal_api/public/sync_manager.h b/sync/internal_api/public/sync_manager.h
index 3f5316b7fd261afad1f61e6165e3e44298d95245..3a9c7a61ca33ff1ec6d24badfa81fc60c8e284a1 100644
--- a/sync/internal_api/public/sync_manager.h
+++ b/sync/internal_api/public/sync_manager.h
@@ -45,6 +45,7 @@ class JsEventHandler;
 class SyncEncryptionHandler;
 class SyncScheduler;
 struct UserShare;
+class CancelationSignal;
 
 namespace sessions {
 class SyncSessionSnapshot;
@@ -320,7 +321,8 @@ class SYNC_EXPORT SyncManager : public syncer::InvalidationHandler {
       Encryptor* encryptor,
       scoped_ptr<UnrecoverableErrorHandler> unrecoverable_error_handler,
       ReportUnrecoverableErrorFunction report_unrecoverable_error_function,
-      bool use_oauth2_token) = 0;
+      bool use_oauth2_token,
+      CancelationSignal* cancelation_signal) = 0;
 
   // Throw an unrecoverable error from a transaction (mostly used for
   // testing).
@@ -392,17 +394,6 @@ class SYNC_EXPORT SyncManager : public syncer::InvalidationHandler {
   // to the syncapi model.
   virtual void SaveChanges() = 0;
 
-  // Initiates shutdown of various components in the sync engine.  Must be
-  // called from the main thread to allow preempting ongoing tasks on the sync
-  // loop (that may be blocked on I/O).  The semantics of |callback| are the
-  // same as with StartConfigurationMode. If provided and a scheduler / sync
-  // loop exists, it will be invoked from the sync loop by the scheduler to
-  // notify that all work has been flushed + cancelled, and it is idle.
-  // If no scheduler exists, the callback is run immediately (from the loop
-  // this was created on, which is the sync loop), as sync is effectively
-  // stopped.
-  virtual void StopSyncingForShutdown() = 0;
-
   // Issue a final SaveChanges, and close sqlite handles.
   virtual void ShutdownOnSyncThread() = 0;