1. allowFileSystem() function clean up in blink. Please refer CLs:
   https://codereview.chromium.org/289793002/ and
   https://codereview.chromium.org/273513005/
2. Use asynchronized api for file system request.

Initial patch.

BUG=343382

Review URL: https://codereview.chromium.org/277353002

git-svn-id: svn://svn.chromium.org/blink/trunk@174764 bbb929c8-8fbe-4397-9dbb-9b2b20218538