If all that mattered was the rendering engine, then Chrome would never have gotten more than 1% browser marketshare, and Firefox would reign supreme. It's pretty obvious to anyone that's paid attention over the last few years that the surrounding UI, Javascript engine, and experience encapsulating the rendering engine is just as important, if not more so.
With Chrome basically the entire UI, including preferences, extensions engine, syncing, and automatic and partial updates are completely open source as part of the Chromium project. Chrome merely puts the Google logo on the cover and packages some pieces of software that can't otherwise be distributed due to licensing constraints. Developers don't consider Ubuntu closed-source just because it has the ability to package closed-source software and drivers with it.
In short: there's no way you can consider Safari an open-source project. There are several ways to classify Chrome as open-source, to which many developers agree.
With Chrome basically the entire UI, including preferences, extensions engine, syncing, and automatic and partial updates are completely open source as part of the Chromium project. Chrome merely puts the Google logo on the cover and packages some pieces of software that can't otherwise be distributed due to licensing constraints. Developers don't consider Ubuntu closed-source just because it has the ability to package closed-source software and drivers with it.
In short: there's no way you can consider Safari an open-source project. There are several ways to classify Chrome as open-source, to which many developers agree.