Remove validating the mount path.

This validation is useless, since it doesn't prevent any abusing, nor errors.

TEST=None
BUG=None

Review URL: https://codereview.chromium.org/84833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237037