While Google at the moment may be committing 50% of the contributions and patches, I would like to point out that Apple has its internal branch of WebKit that they are working on, and when the time comes for a public release there is a ramp up in the WebKit commits on Apple's behalf. This has been documented before.

See here for a nice chart where you can see those jumps in commits: http://techcrunch.com/2010/02/06/google-apple-webkit/