This really depends on the device itself. The number of media key sessions that a device supports is memory dependent. I believe that Android M requirements for media key sessions are something less than 8 but I'm unsure what the specific requirement is. You'll have to check on the specifics of the device you are testing against.

xhwang@, can you take a look if the Nexus 7 device support multi media key session?

this bug also repro on Android O (Pixel XL), xhwang@, can you confirm if multimedia key session is supported on Nexus 7 and Pixel XL?

I tried on Nexus 5x with Android M, and saw an MSE error. +wolenetz for help there.

04-09 15:29:47.498 14678 14678 I chromium: [INFO:CONSOLE(760)] "Uncaught NotSupportedError: Failed to execute 'addSourceBuffer' on 'MediaSource': The type provided ('audio/mp4; codecs="mp4a.40.2"') is unsupported.", source: https://yt-dash-mse-test.commondatastorage.googleapis.com/unit-tests/js/lib/mse/tip/msutil-20180216110138.js (760)

xhwang@ - Since you have a repro, can you try with devtools console on that Android device: what does the following report:
MediaSource.isTypeSupported('audio/mp4; codecs="mp4a.40.2"')

(It should report true IIUC).
Also, please make sure that the Chrome build is built with proprietary_codecs, since that's an AAC codec.

I verified MediaSource.isTypeSupported('audio/mp4; codecs="mp4a.40.2"')
returns true

@#7: Interesting. What does the following report:
document.createElement('audio').canPlayType('audio/mp4; codecs="mp4a.40.2"')

With #7 returning true, I highly expect #8 to return "probably" or "maybe".
Please try this, too:
var a = document.createElement('audio'); var ms = new MediaSource(); ms.onsourceopen=function() { ms.addSourceBuffer('audio/mp4; codecs="mp4a.40.2"'); }; a.src=URL.createObjectURL(ms);

If no exception, then there's no repro of unexpected lack of support for that mime+codec type. Confirm with inspection of resulting ms.sourceBuffers attribute (if empty, addition of the sourcebuffer failed and there should have been an exception).

Sorry for the wrong signal. I think Matt is right that I simply forgot to enable proprietary_codecs in my local build. Now after I enable it, I don't see the MSE error any more.

I'll go ahead to see whether I can repro the bug. But would like to post early here to avoid Matt and Yining wasting more time. Sorry about that.

@#10 Thanks!

I've seem some errors related to provisioning, which may or may not be related to the actual bug Yining is seeing. I'll investigate further until the bug is fixed.

Now issue 834965 has been fixed. Note that this particular test case could still fail due to the fact that the Android device only supports a limited number of MediaDrm instances, and we hit ResourceBusyException [1] when trying to openSession().

[1] https://developer.android.com/reference/android/media/ResourceBusyException