TextureStorage11: Use unique_ptr for RenderTargets. 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>