FileSystem API: Fix memory leak in DirectoryReader, clean up callbacks after reading entries.

BUG=365298
R=kinuko,haraken

Review URL: https://codereview.chromium.org/245193002

git-svn-id: svn://svn.chromium.org/blink/trunk@172131 bbb929c8-8fbe-4397-9dbb-9b2b20218538