Only initialize the timestamp query pools if the extension is available. 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>