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}