There will be a lot of sync mojo IPCs from the renderer, but in single-process mode these will be called from a thread in the browser process. With sync calls disabled in the browser, this means renderer sync IPCs need explicit exceptions as well.

We should just disable the restriction in single-process mode.

Alternatively we could also set the property on a per-thread basis (with a process-wide fallback value), but let's start with the simpler solution.