This cleans up the code. Using QueryHelper means we don't need to
duplicate the timestamp query/result code. It also means we don't need
special allocate/free functions in DynamicQueryPool.

Done while investigating timing GPU events for T-Rex.

Bug: angleproject:4433
Change-Id: I8512a5618e1dd00956942ae2d12d46d8193c4e51
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2081379Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>