Docserver: Add FileSystem.Exists method and start sanity checking path format
via a new path_utils utility.

BUG=none
R=yoz@chromium.org

Review URL: https://codereview.chromium.org/149513014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248730 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248730