Follow up cleaning for https://chromium-review.googlesource.com/c/chromium/src/+/917932

1) We should resolve the typemapping issue for gfx::Size. It's not really clear to me what the limitations of the CrOS side are: apparently ui/gfx is not supposed to be used, but it's really clear to me why (and it's like //media/mojo/interfaces is particularly self-contained).

2) Figure out what is needed to use a more strongly typed platform handle (i.e. handle<shared_buffer> instead of just handle) in CrOS

3) Figure out what is needed to have context associated with mojo callbacks. We really should not be passing arguments through IPC just to be reflected back.