drive: Return cancel closure from FileSystem::GetFileContent

chrome/browser/chromeos/drive/file_system/download_operation.h

Index: chrome/browser/chromeos/drive/file_system/download_operation.h
diff --git a/chrome/browser/chromeos/drive/file_system/download_operation.h b/chrome/browser/chromeos/drive/file_system/download_operation.h
index 7e739f5f9f5a1a7584383fc79778f7cf5b9121b0..19a424056db791422840ad2c2bdc1d22ceac69c1 100644
--- a/chrome/browser/chromeos/drive/file_system/download_operation.h
+++ b/chrome/browser/chromeos/drive/file_system/download_operation.h
@@ -47,7 +47,7 @@ class DownloadOperation {
   ~DownloadOperation();
 
   // Ensures that the file content specified by |local_id| is locally
-  // downloaded.
+  // downloaded and returns a closure to cancel the task.
   // For hosted documents, this method may create a JSON file representing the
   // file.
   // For regular files, if the locally cached file is found, returns it.
@@ -62,7 +62,7 @@ class DownloadOperation {
   // |initialized_callback| and |get_content_callback| can be null if not
   // needed.
   // |completion_callback| must not be null.
-  void EnsureFileDownloadedByLocalId(
+  base::Closure EnsureFileDownloadedByLocalId(
       const std::string& local_id,
       const ClientContext& context,
       const GetFileContentInitializedCallback& initialized_callback,
@@ -71,7 +71,7 @@ class DownloadOperation {
 
   // Does the same thing as EnsureFileDownloadedByLocalId for the file
   // specified by |file_path|.
-  void EnsureFileDownloadedByPath(
+  base::Closure EnsureFileDownloadedByPath(
       const base::FilePath& file_path,
       const ClientContext& context,
       const GetFileContentInitializedCallback& initialized_callback,