Yes, they could have done the same thing with an Android WebView. But the Chrome team is actively working on implementing WebVR (which also want to support many AR use cases) so they already have a native (= higher performance, lower latency, etc) implementation to modify with some smaller changes to use the ARCore API.
There's actually a couple of Chromium-based browsers on there, Samsung Internet for GearVR which has been out and quite popular for 360 video watching (Youtube.. etc.), and Oculus just came out with theirs.
Yes ideally they'd be able to be composited together at the depth buffer level. But even separate I can see a ton of devs making some very cool more UI-centric applications just with CSS mode. Can't wait to see the webdev community understand what can be done with VR and the webstack in 2016.
Really looking forward to the the stereo rendering of non-WebGL content, HTML+CSS, which the Mozilla team is working on. More challenging path to figuring out but will open a lot of possibilities.