For this build:
https://build.chromium.org/p/tryserver.webrtc/builders/linux_libfuzzer_rel/builds/14093

The generate_build_files succeeds, even though it produces an ERROR:
Full logs at https://build.chromium.org/p/tryserver.webrtc/builders/linux_libfuzzer_rel/builds/14093/steps/generate_build_files/logs/stdio

  /b/build/slave/linux64_libfuzzer/build/src/buildtools/linux64/gn gen //out/Release --check
  ERROR at //.gn:40:17: Build argument has no effect.
    mac_sdk_min = "10.11"
                ^------
  The variable "mac_sdk_min" was set as a build argument
  but never appeared in a declare_args() block in any buildfile.

  To view all possible args, run "gn args --list <builddir>"
  Done. Made 384 targets from 131 files in 673ms


It then fails in the compile step, which was called with the error message above as arguments:
Full logs at https://build.chromium.org/p/tryserver.webrtc/builders/linux_libfuzzer_rel/builds/14093/steps/compile/logs/stdio

 cmd: ['/b/build/scripts/slave/.recipe_deps/depot_tools/ninja',
   '-w',
   'dupbuild=err',
   '-C',
   '/b/build/slave/linux64_libfuzzer/build/src/out/Release',
   '-j',
   '50',
   'ERROR',
   'at',
   '//.gn:40:17:',
   'Build',
   'argument',
   'has',
   'no',
   'effect.',
   'mac_sdk_min',
   '=',
   '"10.11"',
   '^------',
   'The',
   'variable',
   '"mac_sdk_min"',
   'was',
   'set',
   'as',
   'a',
   'build',
   'argument',
   'but',
   'never',
   'appeared',
   'in',
   'a',
   'declare_args()',
   'block',
   'in',
   'any',
   'buildfile.',
   'To',
   'view',
   'all',
   'possible',
   'args,',
   'run',
   '"gn',
   'args',
   '--list',
   '<builddir>"',
   'h264_depacketizer_fuzzer',
   'vp8_depacketizer_fuzzer',
   'vp9_depacketizer_fuzzer',
   'vp8_qp_parser_fuzzer',
   'vp9_qp_parser_fuzzer',
   'h264_bitstream_parser_fuzzer',
   'flexfec_header_reader_fuzzer',
   'flexfec_sender_fuzzer',
   'ulpfec_header_reader_fuzzer',
   'ulpfec_generator_fuzzer',
   'flexfec_receiver_fuzzer',
   'packet_buffer_fuzzer',
   'rtcp_receiver_fuzzer',
   'rtp_packet_fuzzer',
   'rtp_header_fuzzer',
   'congestion_controller_feedback_fuzzer',
   'audio_decoder_ilbc_fuzzer',
   'audio_decoder_isac_fuzzer',
   'audio_decoder_isac_incoming_packet_fuzzer',
   'audio_decoder_isacfix_fuzzer',
   'audio_decoder_opus_fuzzer',
   'audio_decoder_opus_redundant_fuzzer',
   'turn_unwrap_fuzzer',
   'neteq_rtp_fuzzer',
   'residual_echo_detector_fuzzer',
   'sdp_parser_fuzzer',
   'stun_parser_fuzzer',
   'stun_validator_fuzzer',
   'pseudotcp_parser_fuzzer',
   'transport_feedback_packet_loss_tracker_fuzzer',
   'audio_processing_fuzzer']


In other bots, the generate build files step simply succeeds without any indication that there was an error.
See https://build.chromium.org/p/tryserver.webrtc/builders/linux_rel/builds/27674/steps/generate_build_files/logs/stdio for example