This allows us to retrieve Device handles for Vulkan, and could allow
us to avoid storing some pointers for other back-ends.

BUG=angleproject:1684

Change-Id: Iccd5b97cf1c3a332d62a38ad0fcf21c352270439
Reviewed-on: https://chromium-review.googlesource.com/426400
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>