Use ExternalFileSystemBackend in the DriveURLRequestJob.

chrome/browser/chromeos/drive/fileapi/file_system_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/drive/fileapi/file_system_backend_delegate.h"
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/drive/file_system_interface.h"
@@ skipping 38 matching lines @@
     const storage::URLCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   FileSystemInterface* const file_system =
       fileapi_internal::GetFileSystemFromUrl(url);
   if (!file_system) {
     BrowserThread::PostTask(
         BrowserThread::IO, FROM_HERE, base::Bind(callback, GURL()));
     return;
   }
   const base::FilePath file_path = util::ExtractDrivePathFromFileSystemUrl(url);
+  if (file_path.empty()) {
+    BrowserThread::PostTask(
+        BrowserThread::IO, FROM_HERE, base::Bind(callback, GURL()));
+    return;
+  }
+
   file_system->GetResourceEntry(
       file_path,
       base::Bind(&GetRedirectURLForContentsOnUIThreadWithResourceEntry,
                  callback));
 }
 
 }  // namespace
 
 FileSystemBackendDelegate::FileSystemBackendDelegate()
     : async_file_util_(new internal::AsyncFileUtil) {
@@ skipping 63 matching lines @@
     const storage::FileSystemURL& url,
     const storage::URLCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   BrowserThread::PostTask(
       BrowserThread::UI,
       FROM_HERE,
       base::Bind(&GetRedirectURLForContentsOnUIThread, url, callback));
 }
 
 }  // namespace drive