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.
Done. Thank you!

FWIW, two of the four examples mentioned in the wiki use single quotes.
wiki page: https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook#some-quick-examples-before-you-get-started

The two examples are etl and git.
https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook#some-quick-examples-before-you-get-started