All the previously implemented surfaces used to preserve the content of
the surface on a swap but this is no longer the case with CGL.

BUG=angleproject:891

Change-Id: I264c579bcbbd22b4eeb09e78aa95751d223694fc
Reviewed-on: https://chromium-review.googlesource.com/289871Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>