Flakiness dashboard: https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#tests=fast%2Fmediastream%2Fdelayed-permission-allowed.html

Flakiness start from wktesthunter data:

r253388                       NOERROR
[r253389-r253396]             UNKNOWN
r253397                       CRASH (Expected: PASS)

Sample stacktrace (same for GTK and WPE):

```
#0  g_type_check_instance_cast () at ../../Source/glib-2.58.1/gobject/gtype.c:4052
#1  0x00007f82fbdcbecf in _ZN7WebCore30WebKitMediaStreamTrackObserver21audioSamplesAvailableERNS_23MediaStreamTrackPrivateERKN3WTF9MediaTimeERKNS_17PlatformAudioDataERKNS_22AudioStreamDescriptionEm () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#2  0x00007f82fce797de in _ZNK7WebCore23MediaStreamTrackPrivate15forEachObserverERKN3WTF8FunctionIFvRNS0_8ObserverEEEE () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#3  0x00007f82fce79b00 in _ZN7WebCore23MediaStreamTrackPrivate21audioSamplesAvailableERKN3WTF9MediaTimeERKNS_17PlatformAudioDataERKNS_22AudioStreamDescriptionEm () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#4  0x00007f82fce7a2ce in _ZNK7WebCore19RealtimeMediaSource15forEachObserverERKN3WTF8FunctionIFvRNS0_8ObserverEEEE () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#5  0x00007f82fce7a7e6 in _ZN7WebCore19RealtimeMediaSource21audioSamplesAvailableERKN3WTF9MediaTimeERKNS_17PlatformAudioDataERKNS_22AudioStreamDescriptionEm () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#6  0x00007f82fd614da2 in _ZN7WebCore27GStreamerAudioCaptureSource17newSampleCallbackEP11_GstElementPS0_ () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#7  0x00007f82f203c8ee in ffi_call_unix64 () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#8  0x00007f82f203c2bf in ffi_call () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#9  0x00007f82f468a966 in g_cclosure_marshal_generic_va () at ../../Source/glib-2.58.1/gobject/gclosure.c:1610
#10 0x00007f82f4689f76 in _g_closure_invoke_va () at ../../Source/glib-2.58.1/gobject/gclosure.c:873
#11 0x00007f82f46a5e27 in g_signal_emit_valist () at ../../Source/glib-2.58.1/gobject/gsignal.c:3300
#12 0x00007f82f46a6452 in g_signal_emit () at ../../Source/glib-2.58.1/gobject/gsignal.c:3447
#13 0x00007f82f550db08 in gst_app_sink_render_common () at ../../Source/gst-plugins-base-1.16.1/gst-libs/gst/app/gstappsink.c:939
#14 0x00007f82f54c2ab2 in gst_base_sink_chain_unlocked () at ../../Source/gstreamer-1.16.1/libs/gst/base/gstbasesink.c:3649
#15 0x00007f82f54c3b50 in gst_base_sink_chain_main () at ../../Source/gstreamer-1.16.1/libs/gst/base/gstbasesink.c:3775
#16 0x00007f82f53cc692 in gst_pad_chain_data_unchecked () at ../../Source/gstreamer-1.16.1/gst/gstpad.c:4327
#17 gst_pad_push_data () at ../../Source/gstreamer-1.16.1/gst/gstpad.c:4583
#18 0x00007f82f53d2e82 in gst_pad_push () at ../../Source/gstreamer-1.16.1/gst/gstpad.c:4702
#19 0x00007f82a077fe45 in gst_queue_push_one () at ../../Source/gstreamer-1.16.1/plugins/elements/gstqueue.c:1384
#20 gst_queue_loop () at ../../Source/gstreamer-1.16.1/plugins/elements/gstqueue.c:1537
#21 0x00007f82f53ffdd1 in gst_task_func () at ../../Source/gstreamer-1.16.1/gst/gsttask.c:328
#22 0x00007f82f45d16a3 in g_thread_pool_thread_proxy () at ../../Source/glib-2.58.1/glib/gthreadpool.c:307
#23 0x00007f82f45d0d45 in g_thread_proxy () at ../../Source/glib-2.58.1/glib/gthread.c:784
#24 0x00007f82f62ecfa3 in start_thread (arg=<optimized out>) at pthread_create.c:486
#25 0x00007f82f2e194cf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
```