Implement GL_KHR_debug. BUG=angleproject:520 Change-Id: I78d14cc8c94f5cef58604220f0ca847473b25bf8 Reviewed-on: https://chromium-review.googlesource.com/317820 Tryjob-Request: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>