Add a FunctionsGLX class to dynamically load the GLX entry points BUG=angleproject:892 Change-Id: I516b7d859100cb065871de8e328585a9ca817499 Reviewed-on: https://chromium-review.googlesource.com/270123 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>