Strip unsused keys from plist.

Remove keys with associated values that have not been replaced by
environment variables. This duplicates the functionality of the xcode
internal plist tool

BUG=236517
TEST=Added pattern match to test/mac/gyptest-app.py
Review URL: https://codereview.chromium.org/14617002/

Patch from Justin Cohen <justincohen@google.com>!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1623 78cadc50-ecff-11dd-a971-7dbc132099af
Only generate output for each configuration once. https://codereview.chromium.org/14139030

git-svn-id: http://gyp.googlecode.com/svn/trunk@1624 78cadc50-ecff-11dd-a971-7dbc132099af