This mostly does nothing except on X11 where it calls the equivalent
glXWait*. eglWait* are needed because they are used by Chromium and
were triggering an UNREACHABLE().

BUG=angleproject:1281

Change-Id: Iff9c127b16841bc27728304a5ba2caff49ff11b5
Reviewed-on: https://chromium-review.googlesource.com/322360
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>