How would you define bare-metal? WebGL is proxied via ANGLE, so there's no risk of crashing or other forms of leaky processes, which is typically the primary thing I think of when I worry about low-level APIs.
What does it mean to sandbox shaders in this context ? GL ES shaders are sent down in a high level language and can't really do much besides do computation on input parameters to generate output in a pipeline. I wish they were more general purpose and worthy of sandboxing but the web GL shaders are really limited.
Second one is about running a bitcoin miner on the GPU, hypothetical memory breaches, and GPU DoS. You can stall the graphics pipeline without shaders, run a miner on CPU, and I'm going to need a demonstration of a usefull GPU memory breach with WebGL. On top of that any such breach is likely driver dependent and will only work on some specific driver/HW combination - that's such a low attack surface I doubt anyone will bother developing exploits targeting that.
I didn't bother going through third because I wasted enough time on first and second, I don't see how anything here suggest you need to sandbox shaders.
[Source: I worked on the original spec and spent a lot of time trying to prevent the more egregious security and privacy problems]
https://news.softpedia.com/news/The-PS4-UI-Is-Built-Using-We...
The only effort to do any GL like stuff was on the PS2, with GL ES 1.0 + Cg, an effort that was dropped when almost no dev cared to use it.
Other than that there was the PSGL library for PS2 Linux.
Anyway, I did a bit more research and Chrome was actually only used for development purposes, they created their own WebGL engine on top of PhyreEngine for the console OS.
https://chromium.googlesource.com/angle/angle/+/master/READM...