These should be using ANGLE_EXPORT, not GL_APICALL.

BUG=angleproject:1523

Change-Id: Ic262656c634209c14ac1e13203f8125c0c65cee5
Reviewed-on: https://chromium-review.googlesource.com/419133Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>