A follow-up CL will make it so the dependencies on
TokenizeExtensionString doesn't pull the GL headers, this will then
allow use to remove the hack for the inclusion of glx.h

BUG=angleproject:892

Change-Id: I51c5466617057e2c580bd5e8056307515bcecc4b
Reviewed-on: https://chromium-review.googlesource.com/272206Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>