This matches the extension spec. Previously we were exposing the ext
as a normal display extension. The extension should work without
needing a display.

Because the extension requires a non-null device for every display we
also add a MockDevice class to handle back-ends which don't implement
any attribute query extensions. By default the device query ext does
not expose any way to use devices so this works fine.

Bug: angleproject:5372
Change-Id: I474310a86aff6a83bd6f9a6b21c8a07c649f306d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2551543Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>