Apparently, Google, Apple, Nokia, and RIM all submit patches to the Webkit project. Google's contributions have surpassed Apple's in recent years..
Edit: I did a little more digging, and it looks like Chrome did start out as a (not very) secret project for a few years. I'm not sure when they started submitting their own patches to Webkit-core (as required by LGPL?), but they did maintain their own fork of webkit until 2009.
They're not required to submit patches, only release code to people you've distributed binaries to. So before Chrome was released, they didn't have to release anything.
> So before Chrome was released, they didn't have to release anything
They did. While LGPL doesn't mean you need to release all your project's code to users if they request it, it does mean you need to release any modifications of the parts covered by the LGPL.
They didn't have to release the changes in any way that made them easy to reintegrate with the mainline version, but they did have to release them somehow if requested to do so by a Chrome user.
Even the full GPL doesn't require you to automatically release your code back to the original source, it only states that you must provide the source to the people you distribute your product too if they request that you do.
Edit: I did a little more digging, and it looks like Chrome did start out as a (not very) secret project for a few years. I'm not sure when they started submitting their own patches to Webkit-core (as required by LGPL?), but they did maintain their own fork of webkit until 2009.