Don't allow path traversal paths on the base file helpers

      This forces explicit normalization of paths and make path escaping security bugs much harder to exploit. See for example bug 167122

BUG=168890
TEST=included tests
Review URL: https://codereview.chromium.org/11782005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175642 0039d316-1c4b-4281-b951-d872f2087c98