Move some more file utils to the base namespace.

This also swaps the order of the parameters to GetShmemTempDir so the out
parameter is last, and enhances some documentation.

Review URL: https://codereview.chromium.org/93263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238144 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238144