Update fuzzer max input length handling

The docs have been updated. max_len is libfuzzer specific, new way is
fuzzer agnostic.

Docs:
https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/getting_started.md#improving-your-fuzz-target

Bug: chromium:895082
Test: flexfec_sender_fuzzer input size still converges at <=200 after running locally for 5-10 minutes.
Change-Id: I7a5ce95cb4d8b8ca461f6e502b81b599daa855f9
Reviewed-on: https://webrtc-review.googlesource.com/c/107883
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25361}

test/fuzzers/rtp_frame_reference_finder_fuzzer.cc

@@ -101,6 +101,9 @@ class FuzzyPacketBuffer : public video_coding::PacketBuffer {
 }  // namespace
 
 void FuzzOneInput(const uint8_t* data, size_t size) {
+  if (size > 20000) {
+    return;
+  }
   DataReader reader(data, size);
   rtc::scoped_refptr<FuzzyPacketBuffer> pb(new FuzzyPacketBuffer(&reader));
   NullCallback cb;