> Increased competition in mobile browsers.

Google is going to capture most of Safari's market share with Chrome. Users know Chrome, they use it on desktop, it's convenient to use it on mobile too.

> Refocus on Android.

With the iPhone market being what it is in the US, I don't see Google letting that just go.

> Apple’s Search Engine

Honestly, I think they just might do that. They have been moving the "service" direction for some time.

> Chromium competition

Microsoft would have made competition for Chrome if they didn't drop their own engine in favour of Chromium.

The more cynical view.