ninja: generalize ExpandProduct()
ninja: generalize ExpandProduct()

This refactoring change moves ExpandProduct in to be a method of
the main class, and renames it to be named ExpandSpecial.  This
is set up for adding another new special variable beyond
$!PRODUCT_DIR.

Review URL: http://codereview.chromium.org/7762001

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp