I can consistently reproduce the error on my local machine.
It doesn't occur with EWS for some reason.


```
stdout:

stderr:
ASSERTION FAILED: Completion handler should always be called
!m_function
/Users/jyavenard/Work/webkit/OpenSource/WebKitBuild/Debug/usr/local/include/wtf/CompletionHandler.h(65) : WTF::CompletionHandler<void (const WTF::MediaTime &)>::~CompletionHandler()
1   0x13904b568 WTFCrash
2   0x119dc226c WTF::CompletionHandler<void (WTF::MediaTime const&)>::~CompletionHandler()
3   0x119dc1238 WTF::CompletionHandler<void (WTF::MediaTime const&)>::~CompletionHandler()
4   0x119dc077c void IPC::handleMessageAsync<Messages::MediaSourcePrivateRemote::SeekToTarget, WebKit::MediaSourcePrivateRemote, WebKit::MediaSourcePrivateRemote, void (WebCore::SeekTarget const&, WTF::CompletionHandler<void (WTF::MediaTime const&)>&&)>(IPC::Connection&, IPC::Decoder&, WebKit::MediaSourcePrivateRemote*, void (WebKit::MediaSourcePrivateRemote::*)(WebCore::SeekTarget const&, WTF::CompletionHandler<void (WTF::MediaTime const&)>&&))
5   0x119dc0538 WebKit::MediaSourcePrivateRemote::didReceiveMessage(IPC::Connection&, IPC::Decoder&)
6   0x11cab597c IPC::MessageReceiverMap::dispatchMessage(IPC::Connection&, IPC::Decoder&)
7   0x11bbb5a18 WebKit::GPUProcessConnection::dispatchMessage(IPC::Connection&, IPC::Decoder&)
8   0x11a6100ec WebKit::GPUProcessConnection::didReceiveMessage(IPC::Connection&, IPC::Decoder&)
9   0x11ca89468 IPC::Connection::dispatchMessage(IPC::Decoder&)
10  0x11ca898fc IPC::Connection::dispatchMessage(std::__1::unique_ptr<IPC::Decoder, std::__1::default_delete<IPC::Decoder>>)
11  0x11ca89c38 IPC::Connection::dispatchOneIncomingMessage()
12  0x11caa85cc IPC::Connection::enqueueIncomingMessage(std::__1::unique_ptr<IPC::Decoder, std::__1::default_delete<IPC::Decoder>>)::$_17::operator()() const
13  0x11caa850c WTF::Detail::CallableWrapper<IPC::Connection::enqueueIncomingMessage(std::__1::unique_ptr<IPC::Decoder, std::__1::default_delete<IPC::Decoder>>)::$_17, void>::call()
14  0x13af95bd4 WTF::Function<void ()>::operator()() const
15  0x139115ae8 WTF::RunLoop::performWork()
16  0x13911a470 WTF::RunLoop::performWork(void*)
17  0x184837d28 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
18  0x184837cbc __CFRunLoopDoSource0
19  0x184837a2c __CFRunLoopDoSources0
20  0x18483661c __CFRunLoopRun
21  0x184835c2c CFRunLoopRunSpecific
22  0x18593e798 -[NSRunLoop(NSRunLoop) runMode:beforeDate:]
23  0x1859b7758 -[NSRunLoop(NSRunLoop) run]
24  0x18448cd64 _xpc_objc_main
25  0x18449ba08 _xpc_main
26  0x18448c910 _xpc_copy_xpcservice_dictionary
27  0x11a7d78b8 WebKit::XPCServiceMain(int, char const**)
28  0x11ca561f0 WKXPCServiceMain
29  0x102e53f9c main
30  0x1843e1058 start
com.apple.WebKit.WebContent.Development terminated (pid 88764) for reason: crash
LEAK: 1 WebPageProxy
```