I'm pretty sure that it's the main line of development and I regularly receive new features, new bugs and new bug fixes as days passes. After some time all this goes to the chrome beta and finally in the chrome release.

I don't think that google adds tons of code 'behind' the back of the opensource repository which is then merged only a few weeks later, because I see a gradual change (aka. the phenotype of those changes) in the daily builds from the chromium repository.