Also add a destroy method.

BUG=angleproject:2464

Change-Id: I7346b799af4e7d64ed5cc3d5eca8e108ce2cf699
Reviewed-on: https://chromium-review.googlesource.com/1054213Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>