Debian does apply patches -- just look at their packaging repository:

http://anonscm.debian.org/cgit/pkg-chromium/pkg-chromium.git...

Google also documents that Chromium may have distro-specific patches applied and how it differs from the Google Chrome binaries provided by Google:

https://code.google.com/p/chromium/wiki/LinuxChromiumPackage...

https://code.google.com/p/chromium/wiki/ChromiumBrowserVsGoo...