1. Google builds new process architecture into Chromium.
1a. Apple asks for it to be contributed back to Webkit.
1b. Google either says "no" or doesn't care but expects Apple to do all the work (note that if Google doesn't do it then this would mean the same functionality being implemented at two different points in the code tree -- webkit and chromium)
2. Apple decides to build its own multiprocess support into Webkit2.
Seems to me that the forking is probably overall a Good Thing, no-one is angry at anybody else, it's just a very important codebase which now has way too many stakeholders.