drive: Return cancel closure from FileSystem::GetFileContent

chrome/browser/chromeos/drive/dummy_file_system.h

Index: chrome/browser/chromeos/drive/dummy_file_system.h
diff --git a/chrome/browser/chromeos/drive/dummy_file_system.h b/chrome/browser/chromeos/drive/dummy_file_system.h
index 464a0e4443fa2cf1d4271ed1d35790089df9e59e..1147f8cffd0a8745d2bb35b844cb501d1862c1fb 100644
--- a/chrome/browser/chromeos/drive/dummy_file_system.h
+++ b/chrome/browser/chromeos/drive/dummy_file_system.h
@@ -58,11 +58,11 @@ class DummyFileSystem : public FileSystemInterface {
                        const GetFileCallback& callback) OVERRIDE {}
   virtual void GetFileForSaving(const base::FilePath& file_path,
                                 const GetFileCallback& callback) OVERRIDE {}
-  virtual void GetFileContent(
+  virtual base::Closure GetFileContent(
       const base::FilePath& file_path,
       const GetFileContentInitializedCallback& initialized_callback,
       const google_apis::GetContentCallback& get_content_callback,
-      const FileOperationCallback& completion_callback) OVERRIDE {}
+      const FileOperationCallback& completion_callback) OVERRIDE;
   virtual void GetResourceEntry(
       const base::FilePath& file_path,
       const GetResourceEntryCallback& callback) OVERRIDE {}