Chrome was initially a completely fork of Webkit, but Google eventually chose to work with Apple and merge all of their changes to the Webkit trunk. This was only possible because Webkit, unlike Android, allows outsiders to commit.
The costs of maintaining a fork of a project as large as Webkit (or Android) are so huge, even Google didn't want to bear them for long. The benefits of the merge are huge for all users. Not only does Chrome continue to get Apple's changes downstream, like CSS transforms, but Safari gets Google's changes, like web sockets. In Android today, this sort of collaboration would be impossible.