[Sync] Try to fix race conditions in CookieJarMismatch.

components/sync/driver/glue/sync_backend_host_impl.cc

Index: components/sync/driver/glue/sync_backend_host_impl.cc
diff --git a/components/sync/driver/glue/sync_backend_host_impl.cc b/components/sync/driver/glue/sync_backend_host_impl.cc
index a15801b563a37fcc4f7d4e8009d06505c5cf73a6..5556b14db9838a689cdd8045c5c4aac31fada5a9 100644
--- a/components/sync/driver/glue/sync_backend_host_impl.cc
+++ b/components/sync/driver/glue/sync_backend_host_impl.cc
@@ -431,11 +431,12 @@ void SyncBackendHostImpl::ClearServerData(
 }
 
 void SyncBackendHostImpl::OnCookieJarChanged(bool account_mismatch,
-                                             bool empty_jar) {
+                                             bool empty_jar,
+                                             const base::Closure& callback) {
   DCHECK(thread_checker_.CalledOnValidThread());
   sync_task_runner_->PostTask(
       FROM_HERE, base::Bind(&SyncBackendHostCore::DoOnCookieJarChanged, core_,
-                            account_mismatch, empty_jar));
+                            account_mismatch, empty_jar, callback));
 }
 
 void SyncBackendHostImpl::ClearServerDataDoneOnFrontendLoop(
@@ -444,6 +445,12 @@ void SyncBackendHostImpl::ClearServerDataDoneOnFrontendLoop(
   frontend_callback.Run();
 }
 
+void SyncBackendHostImpl::OnCookieJarChangedDoneOnFrontendLoop(
+    const base::Closure& callback) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  callback.Run();
+}
+
 }  // namespace syncer
 
 #undef SDVLOG