This function depends on bounds directly -- and now that we've cached bounds, we can theoretically switch this to a cached implementation too.

We could alter this function to be completely opaque:

LayoutDeviceIntPoint RemoteAccessible::ImagePosition(uint32_t aCoordType) {
    return Bounds().TopLeft();
}

... but I think we're probably better off casing off of mCachedFields for now, since there are still slight discrepancies between sync IPC bounds and cached bounds.

Actually I don't think there's any risk to using that ^ version of the function given the sync IPC version just ends up returning Bounds().TopLeft(), which shouldn't change if we fetch Bounds() via sync IPC and then query its TopLeft(). Theoretically sending the point instead of the rect is less IPC traffic, but I don't know how much that affects performance given the overhead of doing IPC at all.
So, nevermind 😀

https://hg.mozilla.org/mozilla-central/rev/d8c1412fe13c