You should start with chrome/installer/mini_installer.sln. Open it up, find the mini_installer target and build it (with component build off). You should have a mini_installer.exe in your build folder that is a self contained installer. Run it with the switches --multi-install --chrome --verbose-logging and it will install chromium for you.
You can build the mini_installer with component build on, but you need to do some trickery to use it: copy *.dll, chrome.7z, setup.exe and setup.exe.manifest to a folder and run setup.exe with the same switches given for mini_installer.