Despite many extensions being written against specific versions of the GL spec,
some drivers (Intel) expose the extensions in much earlier versions.  This
leads to issues where the extension string would be present but the entry points
were not loaded.

BUG=angleproject:1038

Change-Id: Ia87bb3f038f310f6bc3e711a0b2d82717b67ae70
Reviewed-on: https://chromium-review.googlesource.com/286525Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>