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/+/2003230Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>