We had one as well. We're rendering VR with barrel distortion applied in the main vertex shader rather than as a post-process. Saves an expensive framebuffer (some mobile GPUs take as long as half a millisecond to switch render targets) at the cost of a slightly heavier vertex shader. It also requires geometry to be tesselated for the distortion effect to work.
We had to write our own polyfill for our implementation but it beats the post-process technique of webvr-polyfill quite easily.
That's how UE4 and Unity3D do barrel distortion as well.
