Can't enable flac in fMP4 experimental support

[Kyungdahm Yun]

Summary of the bug:
There seems no way to produce a fragmented MP4 containing FLAC audio that I'd like to test with HLS. ffmpeg keeps suggesting -strict -2 to enable experimental support for flac in MP4, but adding this option didn't make any difference.

How to reproduce:

% wget https://storage.googleapis.com/media-session/flac.mp4

% ffmpeg -i flac.mp4 -c copy -strict -2 -hls_segment_type fmp4 -hls_flags single_file flac.m3u8
ffmpeg version 4.2.3 Copyright (c) 2000-2020 the FFmpeg developers
  built with Apple clang version 11.0.3 (clang-1103.0.32.59)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags=-fno-stack-check --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'flac.mp4':
  Metadata:
    major_brand     : iso5
    minor_version   : 512
    compatible_brands: iso6mp41
    encoder         : Lavf57.75.100
  Duration: 00:00:02.74, start: 0.000000, bitrate: 632 kb/s
    Stream #0:0(und): Audio: flac (fLaC / 0x43614C66), 44100 Hz, stereo, s32 (24 bit), 629 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[hls @ 0x7f829e01b800] Opening 'flac.m4s' for writing
[mp4 @ 0x7f829f013600] track 0: codec frame size is not set
[mp4 @ 0x7f829f013600] flac in MP4 support is experimental, add '-strict -2' if you want to use it.
Could not write header for output file #0 (incorrect codec parameters ?): Experimental feature
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
    Last message repeated 1 times

[Kyungdahm Yun]

Never mind. Seems working fine with the current Git master.