We were still calling destroy in checkCompletedCommands. We can
instead call reset.

May not significantly impact performance.

Bug: angleproject:3489
Change-Id: I61a2cb154faac56dc78a49678dd822d81ce16ad2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1647029Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>