sync: Gracefully handle very early shutdown

sync/engine/syncer.cc

Index: sync/engine/syncer.cc
diff --git a/sync/engine/syncer.cc b/sync/engine/syncer.cc
index 3d66ca30fa954d094d07579624dd983e51903d1d..2d67c7f4e7a26a52ca0c1b2eb51c3899397e06ed 100644
--- a/sync/engine/syncer.cc
+++ b/sync/engine/syncer.cc
@@ -19,6 +19,7 @@
 #include "sync/engine/net/server_connection_manager.h"
 #include "sync/engine/process_commit_response_command.h"
 #include "sync/engine/syncer_types.h"
+#include "sync/internal_api/public/base/cancellation_signal.h"
 #include "sync/internal_api/public/base/unique_position.h"
 #include "sync/internal_api/public/util/syncer_error.h"
 #include "sync/sessions/nudge_tracker.h"
@@ -43,20 +44,14 @@ using sessions::StatusController;
 using sessions::SyncSession;
 using sessions::NudgeTracker;
 
-Syncer::Syncer()
-    : early_exit_requested_(false) {
+Syncer::Syncer(syncer::CancellationSignal* cancellation_signal)
+    : cancellation_signal_(cancellation_signal) {
 }
 
 Syncer::~Syncer() {}
 
 bool Syncer::ExitRequested() {
-  base::AutoLock lock(early_exit_requested_lock_);
-  return early_exit_requested_;
-}
-
-void Syncer::RequestEarlyExit() {
-  base::AutoLock lock(early_exit_requested_lock_);
-  early_exit_requested_ = true;
+  return cancellation_signal_->IsStopRequested();
 }
 
 bool Syncer::NormalSyncShare(ModelTypeSet request_types,