Maciej Stachowiak, one of the leaders of Apple’s Safari/WebKit team, in an interesting Hacker News thread on the announcement of Blink:
As long as we are recapitulating history — the main reason we
built a new multiprocess architecture is that Chromium’s
multiprocess support was never contributed to the WebKit
project. It has always lived in the separate Chromium tree,
making it pretty hard to use for non-Chrome purposes. Before we
wrote a single line of what would become WebKit2 we directly
asked Google folks if they would be willing to contribute their
multiprocess support back to WebKit, so that we could build on
it. They said no.
They’ve been on separate paths for a while.