Increase frame history size in VP9 references fuzzer

The previous history size of 32 is likely too small to track long-term
reference frames (such as the initial keyframe). This causes the
circular buffer to overwrite old frames that the encoder may still
reference, leading to an RTC_CHECK failure.

This CL speculatively increases the history size to 256 in an attempt to
prevent the fuzzer from crashing on valid long-term dependencies.

Fixed: chromium:538422136
Change-Id: I964ebaf78bad94176b1ff0068168219c3adf6da8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/494080
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48296}

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 e3546e3..426d598 100644
--- a/test/fuzzers/vp9_encoder_references_fuzzer.cc
+++ b/test/fuzzers/vp9_encoder_references_fuzzer.cc
@@ -93,9 +93,10 @@
                       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.
-  static constexpr size_t kMaxFrameHistorySize = 32;
+  // While 32 frames covers a standard pattern (4 spatial layers * 8 pictures),
+  // fuzzing has shown that references can occasionally reach further back.
+  // 256 is used as a pragmatic buffer to keep older frames in history.
+  static constexpr size_t kMaxFrameHistorySize = 256;
   struct LayerFrame {
     int64_t frame_id;
     int64_t picture_id;