fileSystemProvider: First cut at implementing fileSystemProvider API

chrome/renderer/resources/extensions/file_system_provider_custom_bindings.js

+// 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.
+
+// Custom binding for the fileSystemProvider API.
+
+var binding = require('binding').Binding.create('fileSystemProvider');
+var fileSystemProviderNatives = requireNative('file_system_provider');
+var GetDOMError = fileSystemProviderNatives.GetDOMError;
+
+binding.registerCustomHook(function(bindingsAPI) {
+  var apiFunctions = bindingsAPI.apiFunctions;
+
+  apiFunctions.setUpdateArgumentsPostValidate(
+    'mount',
+    function(displayName, successCallback, errorCallback) {
+      // Piggyback the error callback onto the success callback,
+      // so we can use the error callback later.

[satorux1, 2013/11/07 07:37:02]

kalman: I took another look at other APIs, but could not find a better idea for
holding the error callback. I'm willing to switch to a cleaner solution, so
please give your suggestions. If there is no other easy way, the piggybacking
trick may be acceptable. :P

[not at google - send to devlin, 2013/11/07 15:58:05]

I think the piggybacking trick is fine for now. When we switch over to using
Promise-based APIs we'll get a new error handling mechanism which this will fit
perfectly into.

As a nit, I'd put it on a variable errorCallback_ though (trailing _).

[satorux1, 2013/11/08 02:25:54]

Thanks. Promise-based APIs sound good. Added trailing _ here.

+      successCallback.errorCallback = errorCallback;
+      return [displayName, successCallback];
+    });
+
+  apiFunctions.setCustomCallback(
+    'mount',
+    function(name, request, response) {
+      var fileSystemId = null;
+      var domError = null;
+      if (request.callback && response) {
+        fileSystemId = response[0];
+        // DOMError is present only if mount() failed.
+        if (response[1]) {
+          // Convert a Dictionary to a DOMError.
+          domError = GetDOMError(response[1].name, response[1].message);
+          response.length = 1;
+        }
+
+        var successCallback = request.callback;
+        var errorCallback = request.callback.errorCallback;
+        delete request.callback;
+
+        if (domError)
+          errorCallback(domError);
+        else
+          successCallback(fileSystemId);
+      }
+    });
+});
+
+exports.binding = binding.generate();