ninja/mac: Simplify slightly.

ComputeOutput() does the right thing for bundles, so don't have a distinct
method for bundles. (This also makes bundles honor the 'product_dir' target
key, but I've never seen that used in practice.)

No intended behavior change.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/21428005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1686 78cadc50-ecff-11dd-a971-7dbc132099af