Revert 240799 "[NaCl SDK] nacl_io: Rename Mount -> Filesystem, M..."

Speculative revert to fix 32-bit NaCl failures.

> [NaCl SDK] nacl_io: Rename Mount -> Filesystem, MountNode -> Node.
> 
> (This basically renames everything...)
> 
> BUG=none
> R=noelallen@chromium.org, sbc@chromium.org
> 
> Review URL: https://codereview.chromium.org/113123003

TBR=binji@chromium.org

Review URL: https://codereview.chromium.org/115713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240844 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240844