[fsp] Let extensions decide about the file system id.

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 fileSystemProviderInternal =
     require('binding').Binding.create('fileSystemProviderInternal').generate();
 var eventBindings = require('event_bindings');
@@ skipping 26 matching lines @@
     modificationTime: annotateDate(metadata.modificationTime)
   };
   return result;
 }
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setUpdateArgumentsPostValidate(
     'mount',
-    function(displayName, successCallback, errorCallback) {
+    function(fileSystemId, displayName, successCallback, errorCallback) {
       // Piggyback the error callback onto the success callback,
       // so we can use the error callback later.
       successCallback.errorCallback_ = errorCallback;
-      return [displayName, successCallback];
+      return [fileSystemId, 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]) {
+        if (response[0]) {
           // Convert a Dictionary to a DOMError.
-          domError = GetDOMError(response[1].name, response[1].message);
+          domError = GetDOMError(response[0].name, response[0].message);
           response.length = 1;
         }
 
         var successCallback = request.callback;
         var errorCallback = request.callback.errorCallback_;
         delete request.callback;
 
         if (domError)
           errorCallback(domError);
         else
-          successCallback(fileSystemId);
+          successCallback();
       }
     });
 
   apiFunctions.setUpdateArgumentsPostValidate(
     'unmount',
     function(fileSystemId, successCallback, errorCallback) {
       // Piggyback the error callback onto the success callback,
       // so we can use the error callback later.
       successCallback.errorCallback_ = errorCallback;
       return [fileSystemId, successCallback];
@@ skipping 127 matching lines @@
       };
       var onErrorCallback = function(error) {
         fileSystemProviderInternal.readFileRequestedError(
           fileSystemId, requestId, error);
       }
       dispatch([fileSystemId, openRequestId, offset, length, onSuccessCallback,
                 onErrorCallback]);
     });
 
 exports.binding = binding.generate();