Fix DirectCompositionSurfaceTest.TestMakeCurrent flakiness.

gpu/ipc/service/child_window_win.cc

Index: gpu/ipc/service/child_window_win.cc
diff --git a/gpu/ipc/service/child_window_win.cc b/gpu/ipc/service/child_window_win.cc
index 1246c5773182b4e52ca120b0b9d16df9541e2b87..6c1dfdc957d4c3efcb022157e50f207e79a16896 100644
--- a/gpu/ipc/service/child_window_win.cc
+++ b/gpu/ipc/service/child_window_win.cc
@@ -207,4 +207,9 @@ ChildWindowWin::~ChildWindowWin() {
   }
 }
 
+scoped_refptr<base::TaskRunner> ChildWindowWin::GetTaskRunnerForTesting() {
+  DCHECK(shared_data_);
+  return shared_data_->thread.task_runner();
+}
+
 }  // namespace gpu