This adds the ability for rendering to happen asynchronously. If
objects in-use are deleted as they are being accessed, ownership
is transferred to the Renderer and they are deleted when not in
use. We determine they're ready for delete using a Fence object.

BUG=angleproject:1898

Change-Id: I4fcfd90ad0665d127bf01a10214a604f3407d9e4
Reviewed-on: https://chromium-review.googlesource.com/428353Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>