Adjust check for key frame in vp9_encoder_references_fuzzer

Instead of checking if fuzzer ask encoder to produce a key frame,
check if encoder choose to produce a key frame. In particular that
address scenario where encoding 1st frame that fuzzer ask to be delta
frame, but encoder naturally make it a key frame.

Bug: chromium:441543209
Change-Id: Ia31784500d70caa44ad546ac87d197ba4904c28d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411820
Reviewed-by: Erik Språng <sprang@webrtc.org>
Auto-Submit: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45834}