[SyncFS] Clear sync task runners on shutdown of SyncFileSystemService

chrome/browser/sync_file_system/sync_file_system_service.cc

Index: chrome/browser/sync_file_system/sync_file_system_service.cc
diff --git a/chrome/browser/sync_file_system/sync_file_system_service.cc b/chrome/browser/sync_file_system/sync_file_system_service.cc
index 32a4e21766dab92ce26f5da2821d38d0ba0c7f47..31ba68c6219642a3d3d25ccd3585217c98ef1dde 100644
--- a/chrome/browser/sync_file_system/sync_file_system_service.cc
+++ b/chrome/browser/sync_file_system/sync_file_system_service.cc
@@ -251,6 +251,9 @@ class RemoteSyncRunner : public SyncProcessRunner,
 void SyncFileSystemService::Shutdown() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
+  local_sync_runners_.clear();
+  remote_sync_runners_.clear();
+
   local_service_->Shutdown();
   local_service_.reset();