This issue has been there since beginning of FSP API.

The problem happens when onError callback of any onXXXRequested callbacks is called with OK result code.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0891c24ebc475fb9b92c1d31a7e8d8e49a7de065

commit 0891c24ebc475fb9b92c1d31a7e8d8e49a7de065
Author: mtomasz <mtomasz@chromium.org>
Date: Tue Feb 21 06:36:11 2017

Don't crash browser process when OK passed as error code in FSP API.

This CL fixes it by adding a guard to both custom bindings, and
API implementation.

TEST=Tested with a testing extension.
BUG= 682556 

Review-Url: https://codereview.chromium.org/2643013002
Cr-Commit-Position: refs/heads/master@{#451711}

[modify] https://crrev.com/0891c24ebc475fb9b92c1d31a7e8d8e49a7de065/chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.cc
[modify] https://crrev.com/0891c24ebc475fb9b92c1d31a7e8d8e49a7de065/chrome/common/extensions/api/file_system_provider.idl
[modify] https://crrev.com/0891c24ebc475fb9b92c1d31a7e8d8e49a7de065/chrome/renderer/resources/extensions/file_system_provider_custom_bindings.js