Add advdef / advpng.

advdef / advpng is useful for being able to develop Chromium on OSX.

Chromium instructions pointing to a script that must be run when editing .png resources.
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/app/theme/README

Said script, checking for / using advdef.
https://code.google.com/p/chromium/codesearch#chromium/src/tools/resources/optimize-png-files.sh&q=tools/resources/optimize-png-files.sh&sq=package:chromium&l=467

I imagine that it'd also be useful for Web development, since it plays a part in optimizing .png files.
Can this test be modified to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!