Base: Don't check for thread restrictions when closing a file doesn't do IO.

Also, clarify the File API a little more.

BUG=322664
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/172673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252585 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252585