gdata: Fix a bug that caused a crash when downloading files

chrome/browser/chromeos/gdata/gdata_file_system.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 "chrome/browser/chromeos/gdata/gdata_file_system.h"
 
 #include <errno.h>
 #include <sys/stat.h>
 
 #include <set>
@@ skipping 755 matching lines @@
       FROM_HERE,
       base::Bind(callback, error, file_path, mime_type, file_type));
 }
 
 // Ditto for GetDownloadDataCallback.
 void RelayGetDownloadDataCallback(
     scoped_refptr<base::MessageLoopProxy> relay_proxy,
     const GetDownloadDataCallback& callback,
     GDataErrorCode error,
     scoped_ptr<std::string> download_data) {
+  // Unlike other callbacks, GetDownloadDataCallback is optional, hence it
+  // can be null here.
+  if (callback.is_null())
+    return;
   relay_proxy->PostTask(
       FROM_HERE,
       base::Bind(callback, error, base::Passed(&download_data)));
 }
 
 // Ditto for GetCacheStateCallback.
 void RelayGetCacheStateCallback(
     scoped_refptr<base::MessageLoopProxy> relay_proxy,
     const GetCacheStateCallback& callback,
     base::PlatformFileError error,
@@ skipping 4002 matching lines @@
   pref_registrar_->Init(profile_->GetPrefs());
   pref_registrar_->Add(prefs::kDisableGDataHostedFiles, this);
 }
 
 void SetFreeDiskSpaceGetterForTesting(FreeDiskSpaceGetterInterface* getter) {
   delete global_free_disk_getter_for_testing;  // Safe to delete NULL;
   global_free_disk_getter_for_testing = getter;
 }
 
 }  // namespace gdata