Add Chrome's libpdf and pepper flash binary plugins

Two notes:
* I've added support for Chromium's update.sh script to download the RPMs for the stable channel as well (they are needed for the binary plugins). However, I've not added support for caching the SHA256 hashes of the RPMs, so they are downloaded every time the script is ran.
* pepperflash's version in nixpkgs is equivalent to the version of Chrome from where it was extracted (e.g. "pepperflash-33.0.1750.115", rather than the real version of the Adobe Flash plugin (12.0.0.70).