This ensures they are auto-freed when the Storage is destroyed.
Also remove the 'delete this' design pattern.

BUG=angleproject:2151

Change-Id: I784b94c7125a1bbc15f5d6ae90e55317e199faa6
Reviewed-on: https://chromium-review.googlesource.com/673137
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>