Use proper export label for RequestExtension. These should be using ANGLE_EXPORT, not GL_APICALL. BUG=angleproject:1523 Change-Id: Ic262656c634209c14ac1e13203f8125c0c65cee5 Reviewed-on: https://chromium-review.googlesource.com/419133 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>