Having used ARC, I'm surprised at how well it works. ARC runs applications noticeably faster than my Nexus 5, and it runs amazing well compared to the Android emulator. I think in the next few months well see someone creating a plugin for Android Studio that will allow you to use it as opposed to using the standard emulator with Intel HAXM.
The title is incorrect - it should remove the "OS" part. ARC means "App Runtime for Chrome", it is not specific to Chrome OS.
That isn't a trivial difference - it matters. This is Google bringing Android apps to Chrome - on Mac, on Windows, on Linux, on Chrome OS - and not just to Chrome OS. In other words, Chrome isn't just a browser, it also does things that have nothing to do with being a browser.
Where did you get the Windows, Mac, Linux part? All current ARC apps are Chrome OS only. (e.g.; Vine, Evernote)
Is there a release date for ARC for Win, Mac and Linux?
Is ARC Welder intended to be the official way to do what chromeos-apk[1] does? I have had mixed success with chromeos-apk. Some apps work flawlessly, others crash performing certain operations, and others crash immediately. It would be nice if ARC welder does it all right.
edit: I should have read the other comments first. It looks like ARC does exactly what I'd hoped.[2]
That is an unfortunate name for an API; most people these days know ARC as Automated Reference Counting, which Apple's Cocoa and Cocoa touch use to hide memory management.
Generally, developers focus on a single platform (eg: iOS), and whomever hired them hires another dev to handle the each other mobile platform (eg: Android, etc).
In my experience, I've never seen a same developer work on both.