This bug is obsolete. For context, we were intitially planning to make a version that bypassed the compositor but needed an extra copy to get pixels from the WebGL offscreen canvas onto the transfer Surface/SurfaceTexture, and in that case we'd have needed a separate step to avoid this copy. The current plan is to skip this intermediate step and have WebGL draw directly to the target Surface, see  crbug.com/655722  which tracks this.