<chromium> FIXUP Convert sync points to GL fence syncs

Do not use fence sync with Vivante drivers (i.MX6 SabreLite)
since they are crashing.
Only having glFlush() in GpuCommandBufferStub::OnRetireSyncPoint
seems to be enough for these devices.

Change-Id: I42181e6c2b729f421eebdb6ed31251ad286b303b
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>