Add GetURLForBrowserTab method to the external file system backend.

chrome/browser/chromeos/fileapi/file_system_backend.cc

 // Copyright 2013 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/fileapi/file_system_backend.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/stringprintf.h"
 #include "chrome/browser/chromeos/fileapi/file_access_permissions.h"
@@ skipping 369 matching lines @@
   return scoped_ptr<storage::FileStreamWriter>();
 }
 
 bool FileSystemBackend::GetVirtualPath(
     const base::FilePath& filesystem_path,
     base::FilePath* virtual_path) {
   return mount_points_->GetVirtualPath(filesystem_path, virtual_path) ||
          system_mount_points_->GetVirtualPath(filesystem_path, virtual_path);
 }
 
+void FileSystemBackend::GetAlternativeURL(
+    const storage::FileSystemURL& url,
+    base::Callback<void(const GURL& url)> callback) {
+
+  switch (url.type()) {
+    case storage::kFileSystemTypeDrive:
+      drive_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeProvided:
+      file_system_provider_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeDeviceMediaAsFileStorage:
+      mtp_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeNativeLocal:
+    case storage::kFileSystemTypeRestrictedNativeLocal:
+      callback.Run(GURL());
+      return;
+    default:
+      NOTREACHED();
+      callback.Run(GURL());
+      return;
+  }
+}
+
 }  // namespace chromeos