[NaCl SDK] Fix some directory-reading bugs.

1) MountNodeDir should mark all directories as read/write/execute. We don't
handle access limitations currently.
2) GetDents should always return "." and ".." directories.

BUG=none
R=noelallen@chromium.org

Review URL: https://codereview.chromium.org/24302011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225004 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225004