Why would you consider it fragmentation even if (and that's a big if...not going to happen) Apple allowed Chrome for iOS to use its own version of WebKit (and V8!) instead of UIWebView? That doesn't make any sense.
...Where once there was one rendering engine, now there would be two. That introduces fragmentation for any developers targeting web users on iOS devices, as there would surely be a handful of nuances differentiating the two. Even if those were just around performance.
I don't even think that matters very much, though – I'm just addressing the concern of the crazy fucker above.