[Pepper] Clean up FileSystemResource quota handling

 * Drop max_written_offsets values in FileSystemResource
 * Check FileIO object availability in asynchronously called functions.

BUG=

Review URL: https://codereview.chromium.org/140783008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246592