Specify XrDeviceService sandbox in mojom

The XRDeviceService sandbox is different on Windows than other platforms
so needs to be parameterised in the mojom attribute somehow.

An alternative to this approach is to use #defines in
//sandbox/policy/sandbox_type.h mapping something like kXrDeviceService
to kXrCompositing on Windows and kUtility on other platforms. This
is less invasive to mojom, but also hides that there is a difference
on each platform. This alternative approach will also prevent us from
collapsing the sandbox.mojom.Sandbox and sandbox::policy::SandboxType
enums in future.

Change-Id: I6f1499cd7bb68f612c433794a212c1b92b955f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3041926


Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908690}