Add stub entry points for GL_KHR_debug. BUG=angleproject:520 Change-Id: I6f19ca160214a44c75619c320d5e1db7e4098eda Reviewed-on: https://chromium-review.googlesource.com/317540 Tryjob-Request: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>