These instructions:

  https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reproducing.md

specifically:

  /chromium/src$gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false proprietary_codecs=true'

fails with:

ERROR at //media/BUILD.gn:27:3: Assertion failed.
  assert(
  ^-----
proprietary codecs and ffmpeg_branding set to Chromium are incompatible
See //media/BUILD.gn:28:7: 
      ffmpeg_branding != "Chromium",
      ^----------------------------
This is where it was set.
See //BUILD.gn:278:7: which caused the file to be included.
      "//media:media_unittests",
      ^------------------------