Revert of Base: Make FileProxy automaticaly close the file on a worker thread.

base/files/file_proxy.h

 // Copyright 2014 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.
 
 #ifndef BASE_FILES_FILE_PROXY_H_
 #define BASE_FILES_FILE_PROXY_H_
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/files/file.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 
 namespace tracked_objects {
 class Location;
 };
 
 namespace base {
 
 class TaskRunner;
 class Time;
 
 // This class provides asynchronous access to a File. All methods follow the
 // same rules of the equivalent File method, as they are implemented by bouncing
 // the operation to File using a TaskRunner.
 //
-// This class performs automatic proxying to close the underlying file at
-// destruction.
+// This class does NOT perform automatic proxying to close the underlying file
+// at destruction, which means that it may potentially close the file in the
+// wrong thread (the current thread). If that is not appropriate, the caller
+// must ensure that Close() is called, so that the operation happens on the
+// desired thread.
 //
 // The TaskRunner is in charge of any sequencing of the operations, but a single
 // operation can be proxied at a time, regardless of the use of a callback.
 // In other words, having a sequence like
 //
 //   proxy.Write(...);
 //   delete proxy;
 //
 // will keep the file valid during the Write operation but will cause the file
 // to be closed in the current thread, when the operation finishes. If Close is
@@ skipping 81 matching lines @@
   // This returns false if task posting to |task_runner| has failed.
   bool SetLength(int64 length, const StatusCallback& callback);
 
   // Proxies File::Flush. The callback can be null.
   // This returns false if task posting to |task_runner| has failed.
   bool Flush(const StatusCallback& callback);
 
  private:
   friend class FileHelper;
   void SetFile(File file);
-  TaskRunner* task_runner() { return task_runner_.get(); }
 
   scoped_refptr<TaskRunner> task_runner_;
   File file_;
   DISALLOW_COPY_AND_ASSIGN(FileProxy);
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_PROXY_H_