Expose GL_ANGLE_framebuffer_blit in NULL backend The passthrough ASAN fuzzer was hitting a null-dereference trying to call glBlitFramebuffer when ANGLE was not exposing it. The fuzzer uses the NULL backend so we can just expose it. Bug: chromium:1029829 Change-Id: I0f65c65284756fa21821155559a6404d95e25647 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2003230 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>