Testcase found while fuzzing mozilla-central rev 50df4b75c9b6.

Assertion failure: false (No video encoder for this video track), at /builds/worker/workspace/build/src/dom/media/encoder/MediaEncoder.cpp:568

rax = 0x00005604d86ed180   rdx = 0x0000000000000000
rcx = 0x00007f4ce3f92e1b   rbx = 0x00007f4cbc0e6980
rsi = 0x00007f4cef8c08b0   rdi = 0x00007f4cef8bf680
rbp = 0x00007ffcf5e1b780   rsp = 0x00007ffcf5e1b760
r8 = 0x00007f4cef8c08b0    r9 = 0x00007f4cf0a2a780
r10 = 0x0000000000000000   r11 = 0x0000000000000000
r12 = 0x00007f4cbc3e0800   r13 = 0x00007f4cbc3e0800
r14 = 0x00007f4ca1ea2150   r15 = 0x0000000000000002
rip = 0x00007f4cdfdf93f6
OS|Linux|0.0.0 Linux 4.18.0-25-generic #26~18.04.1-Ubuntu SMP Thu Jun 27 07:28:31 UTC 2019 x86_64
CPU|amd64|family 6 model 94 stepping 3|1
GPU|||
Crash|SIGSEGV|0x0|0
0|0|libxul.so|mozilla::MediaEncoder::ConnectMediaStreamTrack(mozilla::dom::MediaStreamTrack*)|hg:hg.mozilla.org/mozilla-central:dom/media/encoder/MediaEncoder.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|584|0x0
0|1|libxul.so|mozilla::dom::MediaRecorder::Session::InitEncoder(unsigned char, int)|hg:hg.mozilla.org/mozilla-central:dom/media/MediaRecorder.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|978|0x1c
0|2|libxul.so|mozilla::dom::MediaRecorder::Session::MediaStreamReady()|hg:hg.mozilla.org/mozilla-central:dom/media/MediaRecorder.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|802|0x1e
0|3|libxul.so|mozilla::detail::RunnableMethodImpl<mozilla::dom::MediaRecorder::Session*, void (mozilla::dom::MediaRecorder::Session::*)(), true, (mozilla::RunnableKind)0>::Run()|hg:hg.mozilla.org/mozilla-central:xpcom/threads/nsThreadUtils.h:50df4b75c9b6c7fec8c8c4685fd188634d193e75|1176|0x13
0|4|libxul.so|nsThread::ProcessNextEvent(bool, bool*)|hg:hg.mozilla.org/mozilla-central:xpcom/threads/nsThread.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|1224|0x15
0|5|libxul.so|NS_ProcessNextEvent(nsIThread*, bool)|hg:hg.mozilla.org/mozilla-central:xpcom/threads/nsThreadUtils.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|486|0x11
0|6|libxul.so|mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*)|hg:hg.mozilla.org/mozilla-central:ipc/glue/MessagePump.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|88|0xa
0|7|libxul.so|MessageLoop::RunInternal()|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:50df4b75c9b6c7fec8c8c4685fd188634d193e75|315|0x17
0|8|libxul.so|MessageLoop::Run()|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:50df4b75c9b6c7fec8c8c4685fd188634d193e75|290|0x8
0|9|libxul.so|nsBaseAppShell::Run()|hg:hg.mozilla.org/mozilla-central:widget/nsBaseAppShell.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|137|0xd
0|10|libxul.so|nsAppStartup::Run()|hg:hg.mozilla.org/mozilla-central:toolkit/components/startup/nsAppStartup.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|276|0xe
0|11|libxul.so|XREMain::XRE_mainRun()|hg:hg.mozilla.org/mozilla-central:toolkit/xre/nsAppRunner.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|4636|0x11
0|12|libxul.so|XREMain::XRE_main(int, char**, mozilla::BootstrapConfig const&)|hg:hg.mozilla.org/mozilla-central:toolkit/xre/nsAppRunner.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|4771|0x8
0|13|libxul.so|XRE_main(int, char**, mozilla::BootstrapConfig const&)|hg:hg.mozilla.org/mozilla-central:toolkit/xre/nsAppRunner.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|4852|0x5
0|14|firefox-bin|do_main|hg:hg.mozilla.org/mozilla-central:browser/app/nsBrowserApp.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|213|0x22
0|15|firefox-bin|main|hg:hg.mozilla.org/mozilla-central:browser/app/nsBrowserApp.cpp:50df4b75c9b6c7fec8c8c4685fd188634d193e75|295|0xf
0|16|libc-2.27.so||||0x21b97
0|17|firefox-bin|MOZ_ReportCrash|hg:hg.mozilla.org/mozilla-central:mfbt/Assertions.h:50df4b75c9b6c7fec8c8c4685fd188634d193e75|184|0x5

Bug 1014393 is going to rework a whole lot of the code around the media recorder. I wouldn't be surprised if the work in progress there fixed this, but even if it does not, we'll want to wait until the large changes in that bug land before we make other changes.

This crashes with bug 1014393 applied too.

If webm is disabled or not compiled in, the video assert would fail if trying
to record a video track -- we'd only have an ogg/opus encoder.

Depends on D40647

https://hg.mozilla.org/mozilla-central/rev/e4d40d1e47a5
https://hg.mozilla.org/mozilla-central/rev/5007b68ea4b1