ANGLE should build with ARC enabled. I thought Google built ANGLE with ARC enabled, but I see a few -retain and -release calls in ANGLE code, so I'm not sure.
<rdar://problem/87656397>
I'll follow up with Dean or Kyle soon about how upstream builds (unless Kenneth Russell knows), but I don't plan to work on this soon.
Upstream ANGLE (specifically, in Chromium) isn't building with ARC yet. We recently learned from talking with Chrome's Mac development team that it's feasible to turn this on without changing over the entire browser, which makes the problem much more tractable. We'd be glad to work with you to make the switch.