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

base/files/important_file_writer_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/important_file_writer.h"
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
@@ skipping 183 matching lines @@
   file_writer_thread.Start();
   ImportantFileWriter writer(file_, file_writer_thread.task_runner());
   EXPECT_EQ(NOT_CALLED, write_callback_observer_.GetAndResetObservationState());
 
   // Block execution on |file_writer_thread| to verify that callbacks are
   // executed on it.
   base::WaitableEvent wait_helper(
       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"));
   RunLoop().RunUntilIdle();
 
   // Expect the callback to not have been executed before the
   // |file_writer_thread| is unblocked.
   EXPECT_EQ(NOT_CALLED, write_callback_observer_.GetAndResetObservationState());
 
   wait_helper.Signal();
@@ skipping 48 matching lines @@
   writer.ScheduleWrite(&baz);
   ThreadTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE, MessageLoop::QuitWhenIdleClosure(),
       TimeDelta::FromMilliseconds(100));
   RunLoop().Run();
   ASSERT_TRUE(PathExists(writer.path()));
   EXPECT_EQ("baz", GetFileContent(writer.path()));
 }
 
 }  // namespace base