Scoped is changed to DeviceScoped and a ContextScoped class is added.
The former destroys objects at the end of scope, and the latter releases
them.

Bug: angleproject:3757
Change-Id: Ia0bbded53e94af9c8c72c77d07306d6724c6c409
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769060Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>