Add GetURLForBrowserTab method to the external file system backend.

chrome/browser/chromeos/file_system_provider/fileapi/backend_delegate.cc

 // 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.
 
 #include "chrome/browser/chromeos/file_system_provider/fileapi/backend_delegate.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/file_system_provider/fileapi/buffering_file_stream_reader.h"
 #include "chrome/browser/chromeos/file_system_provider/fileapi/file_stream_reader.h"
 #include "chrome/browser/chromeos/file_system_provider/fileapi/file_stream_writer.h"
@@ skipping 51 matching lines @@
   return scoped_ptr<storage::FileStreamWriter>(
       new FileStreamWriter(url, offset));
 }
 
 storage::WatcherManager* BackendDelegate::GetWatcherManager(
     const storage::FileSystemURL& url) {
   NOTIMPLEMENTED();
   return NULL;
 }
 
+void BackendDelegate::GetURLForBrowserTab(
+    const storage::FileSystemURL& url,
+    const URLCallback& callback) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);

[mtomasz, 2014/09/04 05:30:01]

nit: Could you add a check for url type like in #60?

[hirono, 2014/09/04 06:10:58]

Done.

+  callback.Run(GURL());
+}
+
 }  // namespace file_system_provider
 }  // namespace chromeos