[fsp] Simplify aborting logic.

chrome/browser/chromeos/file_system_provider/provided_file_system_interface.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 CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INTERFACE_H_
 #define CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INTERFACE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 39 matching lines @@
   std::string thumbnail;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(EntryMetadata);
 };
 
 // Interface for a provided file system. Acts as a proxy between providers
 // and clients. All of the request methods return an abort callback in order to
 // terminate it while running. They must be called on the same thread as the
 // request methods. The cancellation callback may be null if the operation
-// fails synchronously.
+// fails synchronously. It must not be called once the operation is completed
+// with either a success or an error.
 class ProvidedFileSystemInterface {
  public:
   struct Change;
 
   // Mode of opening a file. Used by OpenFile().
   enum OpenFileMode { OPEN_FILE_MODE_READ, OPEN_FILE_MODE_WRITE };
 
   // Extra fields to be fetched with metadata.
   enum MetadataField {
     METADATA_FIELD_DEFAULT = 0,
@@ skipping 153 matching lines @@
   virtual void RemoveObserver(ProvidedFileSystemObserver* observer) = 0;
 
   // Returns a weak pointer to this object.
   virtual base::WeakPtr<ProvidedFileSystemInterface> GetWeakPtr() = 0;
 };
 
 }  // namespace file_system_provider
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INTERFACE_H_