Add GLX EGL implementation stubs BUG=angleproject:892 Change-Id: I1ef1bd0ce60f4585e473f9750c748a1b41c09da2 Reviewed-on: https://chromium-review.googlesource.com/269741 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>