Allow GYP targets to depend on themselves if they have the
'prune_self_dependencies' set to 1.

See https://codereview.chromium.org/11642018/ for more context.

BUG=chromium:170629

Patch by Alexander Potapenko <glider@chromium.org>
Review URL: https://codereview.appspot.com/7370046/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1589 78cadc50-ecff-11dd-a971-7dbc132099af