Bug 207737

Flaky crashing since it was introduced in r253995

Sample stack from https://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20(Tests)/r256503%20(12649)/imported/w3c/web-platform-tests/media-source/mediasource-h264-play-starved-crash-log.txt

Thread 1 (Thread 0x7f92594639c0 (LWP 41576)):
#0  0x00007f926405325b in _Z24webKitMediaSrcFreeStreamP15_WebKitMediaSrcP7_Stream () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#1  0x00007f92640539df in _Z22webKitMediaSrcFinalizeP8_GObject () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#2  0x00007f925c916cc2 in g_object_unref () at ../../Source/glib-2.58.1/gobject/gobject.c:3346
#3  g_object_unref () at ../../Source/glib-2.58.1/gobject/gobject.c:3238
#4  0x00007f925d64869a in _gst_message_free () at ../../Source/gstreamer-1.16.1/gst/gstmessage.c:212
#5  0x00007f925d621317 in gst_message_unref () at ../../Source/gstreamer-1.16.1/gst/gstmessage.h:382
#6  gst_bus_source_dispatch () at ../../Source/gstreamer-1.16.1/gst/gstbus.c:840
#7  0x00007f925c8316b8 in g_main_dispatch () at ../../Source/glib-2.58.1/glib/gmain.c:3182
#8  g_main_context_dispatch () at ../../Source/glib-2.58.1/glib/gmain.c:3847
#9  0x00007f925c831a78 in g_main_context_iterate () at ../../Source/glib-2.58.1/glib/gmain.c:3920
#10 0x00007f925c831d62 in g_main_loop_run () at ../../Source/glib-2.58.1/glib/gmain.c:4116
#11 0x00007f926164b8e0 in _ZN3WTF7RunLoop3runEv () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libjavascriptcoregtk-4.0.so.18
#12 0x00007f92640405bd in _ZN6WebKit20AuxiliaryProcessMainINS_10WebProcessENS_17WebProcessMainGtkEEEiiPPc () from /home/slave/webkitgtk/gtk-linux-64-release/build/WebKitBuild/Release/lib/libwebkit2gtk-4.0.so.37
#13 0x00007f925afcc09b in __libc_start_main (main=0x5610e5439c00 <main>, argc=4, argv=0x7fff952d61d8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fff952d61c8) at ../csu/libc-start.c:308
#14 0x00005610e5439c7a in _start ()