Rewrite base::Bind into base::BindOnce on trivial cases in base

base/files/important_file_writer_unittest.cc

Index: base/files/important_file_writer_unittest.cc
diff --git a/base/files/important_file_writer_unittest.cc b/base/files/important_file_writer_unittest.cc
index 9b8dcfd4e369d524b86b1851f9a399319f071fb4..88ad35281e61c0270764eda36b0fb0e8c19c231f 100644
--- a/base/files/important_file_writer_unittest.cc
+++ b/base/files/important_file_writer_unittest.cc
@@ -201,8 +201,8 @@ TEST_F(ImportantFileWriterTest, CallbackRunsOnWriterThread) {
       base::WaitableEvent::ResetPolicy::MANUAL,
       base::WaitableEvent::InitialState::NOT_SIGNALED);
   file_writer_thread.task_runner()->PostTask(
-      FROM_HERE,
-      base::Bind(&base::WaitableEvent::Wait, base::Unretained(&wait_helper)));
+      FROM_HERE, base::BindOnce(&base::WaitableEvent::Wait,
+                                base::Unretained(&wait_helper)));
 
   write_callback_observer_.ObserveNextWriteCallbacks(&writer);
   writer.WriteNow(MakeUnique<std::string>("foo"));