This will allow the OpenGL driver to reclaim GPU memory.

BUG=angleproject:1512

Change-Id: If063e1ff9f4a544012ca78c37e198054cb4edcac
Reviewed-on: https://chromium-review.googlesource.com/388771
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>