[fsp] Add requestUnmount() method together with the request manager.

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 2012 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.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
 
@@ skipping 345 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "fileSystem": {
     "dependencies": ["permission:fileSystem"],
     "contexts": ["blessed_extension"]
   },
   "fileSystemProvider": {
     "dependencies": ["permission:fileSystemProvider"],
     "contexts": ["blessed_extension"]
   },
+  "fileSystemProviderInternal": {
+    "internal": true,
+    "dependencies": ["fileSystemProvider"],
+    "contexts": ["blessed_extension", "unblessed_extension"]
+  },
   "firstRunPrivate": {
     "platforms": ["chromeos"],
     "dependencies": ["permission:firstRunPrivate"],
     "contexts": ["blessed_extension"]
   },
   "fontSettings": {
     "dependencies": ["permission:fontSettings"],
     "contexts": ["blessed_extension"]
   },
   "gcm": {
@@ skipping 414 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }