Mac, Linux Leak.

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fthreaded%2Fhttp%2Ftests%2Fworklet%2Fanimation-worklet-import.html&testType=webkit_tests

Sample crash stack:

[32976:14339:1201/065342.729298:16180856645468:FATAL:weak_ptr.cc(26)] Check failed: sequence_checker_.CalledOnValidSequence(). WeakPtrs must be checked on the same sequenced thread.
0   Content Shell Framework             0x000000010e07c9a3 base::debug::StackTrace::StackTrace() + 19
1   Content Shell Framework             0x000000010e09fb47 logging::LogMessage::~LogMessage() + 71
2   Content Shell Framework             0x000000010e0b1db7 base::internal::WeakReference::is_valid() const + 119
3   Content Shell Framework             0x000000011148d890 blink::ThreadedWorkletObjectProxy::reportConsoleMessage(blink::MessageSource, blink::MessageLevel, WTF::String const&, blink::SourceLocation*) + 48
4   Content Shell Framework             0x00000001110223b4 blink::WorkerThreadDebugger::exceptionThrown(blink::WorkerThread*, blink::ErrorEvent*) + 84
5   Content Shell Framework             0x000000011148c98e blink::ThreadedWorkletGlobalScope::exceptionThrown(blink::ErrorEvent*) + 158
6   Content Shell Framework             0x0000000110a21151 blink::ExecutionContext::dispatchErrorEvent(blink::ErrorEvent*, blink::AccessControlStatus) + 225
7   Content Shell Framework             0x00000001101f8a89 blink::WorkerOrWorkletScriptController::evaluate(blink::ScriptSourceCode const&, blink::ErrorEvent**, blink::CachedMetadataHandler*, blink::V8CacheOptions) + 985
8   Content Shell Framework             0x000000011148d100 blink::(anonymous namespace)::evaluateScriptOnWorkletGlobalScope(WTF::String const&, blink::KURL const&, blink::ExecutionContext*) + 192
9   Content Shell Framework             0x00000001112929f0 blink::internal::CallClosureTask<void (blink::ExecutionContext*), (WTF::FunctionThreadAffinity)0>::performTask(blink::ExecutionContext*) + 32
10  Content Shell Framework             0x00000001114a3b5a blink::WorkerThread::performTaskOnWorkerThread(std::__1::unique_ptr<blink::ExecutionContextTask, std::__1::default_delete<blink::ExecutionContextTask> >, bool) + 362
11  Content Shell Framework             0x00000001114a8a6e base::internal::Invoker<base::internal::BindState<void (blink::WorkerThread::*)(std::__1::unique_ptr<blink::ExecutionContextTask, std::__1::default_delete<blink::ExecutionContextTask> >, bool), WTF::UnretainedWrapper<blink::WorkerThread, (WTF::FunctionThreadAffinity)0>, WTF::PassedWrapper<std::__1::unique_ptr<blink::ExecutionContextTask, std::__1::default_delete<blink::ExecutionContextTask> > >, bool>, void ()>::Run(base::internal::BindStateBase*) + 62
12  Content Shell Framework             0x000000010e07d0c1 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) + 225
13  Content Shell Framework             0x000000010e0b5e19 base::MessageLoop::RunTask(base::PendingTask*) + 425
14  Content Shell Framework             0x000000010e0b61dc base::MessageLoop::DeferOrRunPendingTask(base::PendingTask) + 44
15  Content Shell Framework             0x000000010e0b66d3 base::MessageLoop::DoWork() + 467
16  Content Shell Framework             0x000000010e0ba717 base::MessagePumpCFRunLoopBase::RunWork() + 55
17  Content Shell Framework             0x000000010e0a08ba base::mac::CallWithEHFrame(void ( block_pointer)()) + 10
18  Content Shell Framework             0x000000010e0ba0e4 base::MessagePumpCFRunLoopBase::RunWorkSource(void*) + 68
19  CoreFoundation                      0x00007fff85810881 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
20  CoreFoundation                      0x00007fff857effbc __CFRunLoopDoSources0 + 556
21  CoreFoundation                      0x00007fff857ef4df __CFRunLoopRun + 927
22  CoreFoundation                      0x00007fff857eeed8 CFRunLoopRunSpecific + 296
23  Content Shell Framework             0x000000010e0bac4f base::MessagePumpCFRunLoop::DoRun(base::MessagePump::Delegate*) + 79
24  Content Shell Framework             0x000000010e0ba567 base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*) + 119
25  Content Shell Framework             0x000000010e0b5b62 base::MessageLoop::RunHandler() + 354
26  Content Shell Framework             0x000000010e0e71f3 base::RunLoop::Run() + 51
27  Content Shell Framework             0x000000010e12b7a7 base::Thread::Run(base::RunLoop*) + 199
28  Content Shell Framework             0x000000010e12bd16 base::Thread::ThreadMain() + 854
29  Content Shell Framework             0x000000010e12238f base::(anonymous namespace)::ThreadFunc(void*) + 95
30  libsystem_pthread.dylib             0x00007fff95fa399d _pthread_body + 131
31  libsystem_pthread.dylib             0x00007fff95fa391a _pthread_body + 0
32  libsystem_pthread.dylib             0x00007fff95fa1351 thread_start + 13

Looks like WeakPtr is being used in a way that isn't thread-safe. Will land a CL to skip this for now.