Revert 237037 "Remove validating the mount path."

Because of reverting 237034, we need to revert this patch too.

> 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

TBR=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/85283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237039 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237039