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.