UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.0 Safari/537.36

Example URL:
https://webrtc.github.io/samples/src/content/getusermedia/record/

Steps to reproduce the problem:
1. go to https://webrtc.github.io/samples/src/content/getusermedia/record/
2. open dev tools
3. put a breakpoint here:

function startRecording() {
  var options = {mimeType: 'video/webm'};
  recordedBlobs = [];  // BREAKPOINT HERE

4. hit "start recording'; you'll hit aforementioned breakpoint
5. in the console, type options.mimeType = 'video/webm;codecs=vp9,opus'
6. continue program execution.  Record some video.
7. Stop recording, then try to playback.  No video will be shown.  Audio may work.

If you download the file and examine it with ffprobe, you'll see:

Jeremys-MacBook-Pro-2:~ jnoring$ ffprobe ~/Downloads/chrome_52_broken.webm
ffprobe version 3.0 Copyright (c) 2007-2016 the FFmpeg developers
  built with Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-libfontconfig --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfaac --enable-libass --enable-ffplay --enable-libspeex --enable-libfdk-aac --enable-libopus --enable-frei0r --enable-libcaca --enable-libsoxr --enable-libx265 --enable-libbs2b --enable-nonfree --enable-vda
  libavutil      55. 17.103 / 55. 17.103
  libavcodec     57. 24.102 / 57. 24.102
  libavformat    57. 25.100 / 57. 25.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 31.100 /  6. 31.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Input #0, matroska,webm, from '/Users/jnoring/Downloads/chrome_52_broken.webm':
  Metadata:
    encoder         : Chrome
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng): Audio: opus, 48000 Hz, mono, fltp (default)
    Stream #0:1(eng): Video: vp8, yuv420p, 640x480, SAR 1:1 DAR 4:3, 24 fps, 24 tbr, 1k tbn, 1k tbc (default)

...it looks like the super blob (which is a single blob consisting of all the smaller blobs) thinks it's vp8, which it isn't.  If the file is played back, there's complaints about VP8 header size being larger than data provided, which is likely because it should be played with vp9?

What is the expected behavior?
Blob can be played back locally, or downloaded and played.

What went wrong?
I believe the resulting blob is corrupted and/or incorrect, and cannot be played back.

Did this work before? Yes Chrome Beta (51) works fine, as does Chrome 50.

Is it a problem with Flash or HTML5? HTML5

Does this work in other browsers? Yes 

Chrome version: 52.0.2743.0  Channel: canary
OS Version: OS X 10.9.5
Flash Version: Shockwave Flash 22.0 r0