Use ExternalFileSystemBackend in the DriveURLRequestJob.

chrome/browser/chromeos/fileapi/file_system_backend.cc

Index: chrome/browser/chromeos/fileapi/file_system_backend.cc
diff --git a/chrome/browser/chromeos/fileapi/file_system_backend.cc b/chrome/browser/chromeos/fileapi/file_system_backend.cc
index 568a9c586fdce147e68ed3be2cb5956bc59eaf12..5e715771dbae9b1ff418b1ee14f085502c516dc1 100644
--- a/chrome/browser/chromeos/fileapi/file_system_backend.cc
+++ b/chrome/browser/chromeos/fileapi/file_system_backend.cc
@@ -10,6 +10,7 @@
 #include "chrome/browser/chromeos/fileapi/file_access_permissions.h"
 #include "chrome/browser/chromeos/fileapi/file_system_backend_delegate.h"
 #include "chrome/browser/media_galleries/fileapi/media_file_system_backend.h"
+#include "chrome/common/url_constants.h"
 #include "chromeos/dbus/cros_disks_client.h"
 #include "storage/browser/blob/file_stream_reader.h"
 #include "storage/browser/fileapi/async_file_util.h"
@@ -180,6 +181,9 @@ bool FileSystemBackend::IsAccessAllowed(
     return true;
   }
 
+  if (url.origin().scheme() == chrome::kDriveScheme)

[mtomasz, 2014/09/19 05:30:53]

nit: We should either have a test that such url can't be passed from an
extension, or at least a comment why it's safe.

[hirono, 2014/09/19 06:26:36]

I added a comment. Let me add a test to prohibit parsing
filesystem:externalfile:/xxx, in the future CL. I'm planning to add it to
external_file_url_util.cc.
https://codereview.chromium.org/580023003/patch/20001/30017

+    return true;
+
   // Check first to make sure this extension has fileBrowserHander permissions.
   if (!special_storage_policy_.get() ||
       !special_storage_policy_->IsFileHandler(extension_id))