Add OnFrameDropped override to vp9 encoder fuzzer.

Bug: webrtc:467444018
Change-Id: Ia1d0b220f6370479baeafb2de7fae4533ad096b0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470540
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47650}

test/fuzzers/vp9_encoder_references_fuzzer.cc

diff --git a/test/fuzzers/vp9_encoder_references_fuzzer.cc b/test/fuzzers/vp9_encoder_references_fuzzer.cc
index cb84d5c..44359bd 100644
--- a/test/fuzzers/vp9_encoder_references_fuzzer.cc
+++ b/test/fuzzers/vp9_encoder_references_fuzzer.cc
@@ -88,6 +88,10 @@
     return Result(Result::OK);
   }
 
+  void OnFrameDropped(uint32_t rtp_timestamp,
+                      int spatial_id,
+                      bool is_end_of_temporal_unit) override {}
+
  private:
   // With 4 spatial layers and patterns up to 8 pictures, it should be enough to
   // keep the last 32 frames to validate dependencies.