Also added a few assertions to ensure that timestamp queries aren't attempted
when support is missing.

Bug: angleproject:4114
Change-Id: Ie6d7d5face59f9bc137aebd86c9d0e965773e6e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1992184
Commit-Queue: Eric Binet <ericbinet@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>