[SyncFS] Clear sync task runners on shutdown of SyncFileSystemService

This CL should fix a memory corruption in a unit_tests which is recently introduced.
That is caused by sync task invocation after SyncFileSystemService::Shutdown.

BUG=394460
TEST=unit_tests --gtest_filter='SyncFileSystemServiceTest.*'

Review URL: https://codereview.chromium.org/395333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283756 0039d316-1c4b-4281-b951-d872f2087c98

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();
 

tools/valgrind/gtest_exclude/unit_tests.gtest-drmemory.txt

-# Flaky, see http://crbug.com/394460
-SyncFileSystemServiceTest.SimpleLocalSyncFlow

tools/valgrind/gtest_exclude/unit_tests.gtest-memcheck_linux.txt

-# Flaky, see http://crbug.com/394460
-SyncFileSystemServiceTest.SimpleLocalSyncFlow